<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2023 Workiva-->
<!--r:c5554fc9-877a-417c-ba1e-d85c98ff0a37,g:f756d813-5532-4081-8c62-3c28568a5306-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://momentus.space/role/CoverPage" xlink:type="simple" xlink:href="mnts-20230930.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://momentus.space/role/CoverPage" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_635dece0-6e28-4eea-9eae-99756c86b075" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentType_635dece0-6e28-4eea-9eae-99756c86b075" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_20a39de6-5546-430b-8f99-11d71354eb83" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentQuarterlyReport_20a39de6-5546-430b-8f99-11d71354eb83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_c90b67d0-702e-4f12-a6de-2b040b23c37c" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentPeriodEndDate_c90b67d0-702e-4f12-a6de-2b040b23c37c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_6785642e-f7a1-4897-a3ad-49b8f859d933" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentTransitionReport_6785642e-f7a1-4897-a3ad-49b8f859d933" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_1ff54d9d-6b41-45a4-827c-bc32daf3fdd9" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityFileNumber_1ff54d9d-6b41-45a4-827c-bc32daf3fdd9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_9845912e-e9af-4c2b-91df-0e31c0312eb0" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityRegistrantName_9845912e-e9af-4c2b-91df-0e31c0312eb0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_45e3fd9f-2f2a-46b7-bd5c-6cd332c1caff" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityIncorporationStateCountryCode_45e3fd9f-2f2a-46b7-bd5c-6cd332c1caff" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_4c6ff0b7-47aa-46d5-b506-d1b9e43901ee" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityTaxIdentificationNumber_4c6ff0b7-47aa-46d5-b506-d1b9e43901ee" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_79c4dd68-1362-4733-b318-9fc46eb2f0b2" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityAddressAddressLine1_79c4dd68-1362-4733-b318-9fc46eb2f0b2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_0bacbce7-26e3-4574-9265-5d8c86d9f1b6" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityAddressCityOrTown_0bacbce7-26e3-4574-9265-5d8c86d9f1b6" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_9bda31ab-71ed-4dd2-8e5d-af85063c2a8a" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityAddressStateOrProvince_9bda31ab-71ed-4dd2-8e5d-af85063c2a8a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_02c66846-4a9e-458b-9674-b9a9a2413395" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityAddressPostalZipCode_02c66846-4a9e-458b-9674-b9a9a2413395" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_625d9029-3a8b-4ccc-b218-e4184a6afe29" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_CityAreaCode_625d9029-3a8b-4ccc-b218-e4184a6afe29" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_5496dc33-c777-43a1-895b-cbcea7037c98" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_LocalPhoneNumber_5496dc33-c777-43a1-895b-cbcea7037c98" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_7e70305a-763f-44e8-a1fc-5b46adcaa3bc" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_Security12bTitle_7e70305a-763f-44e8-a1fc-5b46adcaa3bc" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_493795c1-5186-49ab-a31f-5fb6bbcf425a" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_TradingSymbol_493795c1-5186-49ab-a31f-5fb6bbcf425a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_ea7f6994-5f75-4f6c-b191-dae8260ab37e" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_SecurityExchangeName_ea7f6994-5f75-4f6c-b191-dae8260ab37e" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_7a91c201-0568-4a92-957f-1baaf58555ef" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityCurrentReportingStatus_7a91c201-0568-4a92-957f-1baaf58555ef" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_1cb32690-d196-4f38-9a3f-8b1aaaee78b4" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityInteractiveDataCurrent_1cb32690-d196-4f38-9a3f-8b1aaaee78b4" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_0c3271b3-d519-4c83-9321-c9825b8819e8" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityFilerCategory_0c3271b3-d519-4c83-9321-c9825b8819e8" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_e33713fd-01ff-47fb-8162-05225eebbf26" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntitySmallBusiness_e33713fd-01ff-47fb-8162-05225eebbf26" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_68f82915-dbb5-4c96-b260-0a1cbf34491a" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityEmergingGrowthCompany_68f82915-dbb5-4c96-b260-0a1cbf34491a" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_9d047746-3af2-4590-8c08-c8386db2d563" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityExTransitionPeriod_9d047746-3af2-4590-8c08-c8386db2d563" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_3b3b10a0-2857-4f6c-9150-8881c97f042d" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityShellCompany_3b3b10a0-2857-4f6c-9150-8881c97f042d" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_cd15591e-0065-448d-a309-4f3c4deeff61" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_cd15591e-0065-448d-a309-4f3c4deeff61" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_36610569-37e7-46a4-bcd5-2027357914c1" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_EntityCentralIndexKey_36610569-37e7-46a4-bcd5-2027357914c1" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_516ac4b8-d1b2-4c7d-9dbb-26ba59825925" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentFiscalYearFocus_516ac4b8-d1b2-4c7d-9dbb-26ba59825925" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_07159be8-2c62-4fb1-b05f-6d02da1d7931" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentFiscalPeriodFocus_07159be8-2c62-4fb1-b05f-6d02da1d7931" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_e9689878-2996-4c42-a610-d17560477792" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_AmendmentFlag_e9689878-2996-4c42-a610-d17560477792" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_e2d6fcb7-73dd-413d-af56-388395d6373a" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_CurrentFiscalYearEndDate_e2d6fcb7-73dd-413d-af56-388395d6373a" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_cca6d9e3-7ee1-42ec-a930-663b9a6c3388" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_ee3c8654-a3e8-40a4-b039-c3bba7674145" xlink:to="loc_dei_DocumentInformationTable_cca6d9e3-7ee1-42ec-a930-663b9a6c3388" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_cca6d9e3-7ee1-42ec-a930-663b9a6c3388" xlink:to="loc_us-gaap_StatementClassOfStockAxis_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6" xlink:to="loc_us-gaap_ClassOfStockDomain_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f80d099a-7c1a-414f-9c89-fdb796228a0a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6ef43e2f-f12a-4e7f-9b03-9f20d6ee43e6" xlink:to="loc_us-gaap_ClassOfStockDomain_f80d099a-7c1a-414f-9c89-fdb796228a0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_e616a505-f6c6-4c29-a4c6-10f67a942441" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f80d099a-7c1a-414f-9c89-fdb796228a0a" xlink:to="loc_us-gaap_CommonClassAMember_e616a505-f6c6-4c29-a4c6-10f67a942441" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_cb3f5ea3-d2e8-48e0-9ad2-73e6e30a5756" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f80d099a-7c1a-414f-9c89-fdb796228a0a" xlink:to="loc_us-gaap_WarrantMember_cb3f5ea3-d2e8-48e0-9ad2-73e6e30a5756" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="mnts-20230930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://momentus.space/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_c12feaa8-0d7e-4ce3-a384-71e0f3ea2d7c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c12feaa8-0d7e-4ce3-a384-71e0f3ea2d7c" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_c7ddf526-46c3-401d-aeb1-57062d1b96ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_c7ddf526-46c3-401d-aeb1-57062d1b96ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a0254fda-badd-46ab-baf5-91cccd83ee56" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockholdersEquity_a0254fda-badd-46ab-baf5-91cccd83ee56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_3c42fee5-9f6e-4345-88e5-b64987b368df" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_3c42fee5-9f6e-4345-88e5-b64987b368df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e690591d-1a32-431c-81b0-6126ae0a47cf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e690591d-1a32-431c-81b0-6126ae0a47cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_7219c41a-0c24-4fe0-849d-41ea5056900e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_7219c41a-0c24-4fe0-849d-41ea5056900e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_59872b06-9aa5-4740-a32b-9a7a842556b7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_59872b06-9aa5-4740-a32b-9a7a842556b7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_a1cf1cfc-ed89-4177-9039-2e96da2510b1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_a1cf1cfc-ed89-4177-9039-2e96da2510b1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_StockIssuedDuringPeriodSharesSection16OfficerTaxCoverageExchange_8cd436c2-7030-42fc-b290-ada6b135f167" xlink:href="mnts-20230930.xsd#mnts_StockIssuedDuringPeriodSharesSection16OfficerTaxCoverageExchange"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_mnts_StockIssuedDuringPeriodSharesSection16OfficerTaxCoverageExchange_8cd436c2-7030-42fc-b290-ada6b135f167" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_StockIssuedDuringPeriodValueSection16OfficerTaxCoverageExchange_292c6cca-7da1-4eb9-8057-826f14c76e20" xlink:href="mnts-20230930.xsd#mnts_StockIssuedDuringPeriodValueSection16OfficerTaxCoverageExchange"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_mnts_StockIssuedDuringPeriodValueSection16OfficerTaxCoverageExchange_292c6cca-7da1-4eb9-8057-826f14c76e20" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_StockIssuedDuringPeriodSharesPurchaseOfPreFundedWarrants_f2923ddc-a812-41fa-b597-f5ae838e73b3" xlink:href="mnts-20230930.xsd#mnts_StockIssuedDuringPeriodSharesPurchaseOfPreFundedWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_mnts_StockIssuedDuringPeriodSharesPurchaseOfPreFundedWarrants_f2923ddc-a812-41fa-b597-f5ae838e73b3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_7378f89c-6e8f-44f7-9039-b2ccb57b710a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_7378f89c-6e8f-44f7-9039-b2ccb57b710a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalOther_a084c48a-e42c-4ec2-8227-8b7b37a73411" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalOther_a084c48a-e42c-4ec2-8227-8b7b37a73411" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_05fe3f40-bb40-4cf8-95b9-c0c260436da9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_05fe3f40-bb40-4cf8-95b9-c0c260436da9" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_d6cc1215-7888-4ec4-b5d4-52e2799d6741" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_d6cc1215-7888-4ec4-b5d4-52e2799d6741" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_ffa1656e-5fef-465c-b1a5-4519822ef224" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_ffa1656e-5fef-465c-b1a5-4519822ef224" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_24268fdf-bda7-46b7-9b04-7465d902b23b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_24268fdf-bda7-46b7-9b04-7465d902b23b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_57d08967-1556-40a7-855c-db883e67d691" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_57d08967-1556-40a7-855c-db883e67d691" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits_acca4c57-ad09-4c67-81da-8c856bea1bd6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits_acca4c57-ad09-4c67-81da-8c856bea1bd6" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9cce053d-c323-43d1-9f34-9755a54d86d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fb3aacb1-2c4e-40e7-bff7-388d6b492554" xlink:to="loc_us-gaap_NetIncomeLoss_9cce053d-c323-43d1-9f34-9755a54d86d0" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_1981e096-48fd-4364-a8f7-a91e00162c2c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_d4ab3262-337a-4d5e-81fd-724db3dfd411" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_e17ca3a2-1034-4e55-b720-cb14fbf40057" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_c12feaa8-0d7e-4ce3-a384-71e0f3ea2d7c" xlink:to="loc_us-gaap_StatementTable_e17ca3a2-1034-4e55-b720-cb14fbf40057" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_c4a3ac55-8548-4297-abdb-ebc42b752f15" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e17ca3a2-1034-4e55-b720-cb14fbf40057" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_c4a3ac55-8548-4297-abdb-ebc42b752f15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c4a3ac55-8548-4297-abdb-ebc42b752f15_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c4a3ac55-8548-4297-abdb-ebc42b752f15" xlink:to="loc_us-gaap_EquityComponentDomain_c4a3ac55-8548-4297-abdb-ebc42b752f15_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_8d502b1f-3de6-4841-b685-689421c1d69b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c4a3ac55-8548-4297-abdb-ebc42b752f15" xlink:to="loc_us-gaap_EquityComponentDomain_8d502b1f-3de6-4841-b685-689421c1d69b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_32699f63-2818-4cd7-b75f-bb858e1d6a06" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8d502b1f-3de6-4841-b685-689421c1d69b" xlink:to="loc_us-gaap_CommonStockMember_32699f63-2818-4cd7-b75f-bb858e1d6a06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_3bed5204-d4b5-44fb-ab42-28d92d01a214" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8d502b1f-3de6-4841-b685-689421c1d69b" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_3bed5204-d4b5-44fb-ab42-28d92d01a214" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_24e394f9-4ca2-45a0-ba06-62e62ccd8750" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8d502b1f-3de6-4841-b685-689421c1d69b" xlink:to="loc_us-gaap_RetainedEarningsMember_24e394f9-4ca2-45a0-ba06-62e62ccd8750" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_0b1e46de-5d1c-48c7-b18d-b8181b32a989" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e17ca3a2-1034-4e55-b720-cb14fbf40057" xlink:to="loc_us-gaap_StatementClassOfStockAxis_0b1e46de-5d1c-48c7-b18d-b8181b32a989" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0b1e46de-5d1c-48c7-b18d-b8181b32a989_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0b1e46de-5d1c-48c7-b18d-b8181b32a989" xlink:to="loc_us-gaap_ClassOfStockDomain_0b1e46de-5d1c-48c7-b18d-b8181b32a989_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9dfa650a-7888-4061-9584-c762ae759fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0b1e46de-5d1c-48c7-b18d-b8181b32a989" xlink:to="loc_us-gaap_ClassOfStockDomain_9dfa650a-7888-4061-9584-c762ae759fe0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_1e5b5e12-397c-44ff-a7ae-a94cea74994b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9dfa650a-7888-4061-9584-c762ae759fe0" xlink:to="loc_us-gaap_CommonClassAMember_1e5b5e12-397c-44ff-a7ae-a94cea74994b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/NatureofOperationsDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#NatureofOperationsDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/NatureofOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:href="mnts-20230930.xsd#mnts_ScheduleOfReverseRecapitalizationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfCustomerSatellitesDeployedInLowEarthOrbit_eb4d24fa-7013-44e2-9b03-53935c3b7009" xlink:href="mnts-20230930.xsd#mnts_NumberOfCustomerSatellitesDeployedInLowEarthOrbit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfCustomerSatellitesDeployedInLowEarthOrbit_eb4d24fa-7013-44e2-9b03-53935c3b7009" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfInternalCustomerSatellitesDeployedInLowEarthOrbit_3857eb37-3df1-4c95-a6dc-70edfdaec339" xlink:href="mnts-20230930.xsd#mnts_NumberOfInternalCustomerSatellitesDeployedInLowEarthOrbit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfInternalCustomerSatellitesDeployedInLowEarthOrbit_3857eb37-3df1-4c95-a6dc-70edfdaec339" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfThirdPartyCustomerSatellitesDeployedInLowEarthOrbit_ad093e12-5d3a-4e93-aa13-5abc34fd9f99" xlink:href="mnts-20230930.xsd#mnts_NumberOfThirdPartyCustomerSatellitesDeployedInLowEarthOrbit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfThirdPartyCustomerSatellitesDeployedInLowEarthOrbit_ad093e12-5d3a-4e93-aa13-5abc34fd9f99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfCustomerPayloads_8a6f7804-11d2-477a-813e-bc448296393a" xlink:href="mnts-20230930.xsd#mnts_NumberOfCustomerPayloads"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfCustomerPayloads_8a6f7804-11d2-477a-813e-bc448296393a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfPatents_27042786-8189-4168-8599-23cee277d627" xlink:href="mnts-20230930.xsd#mnts_NumberOfPatents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfPatents_27042786-8189-4168-8599-23cee277d627" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfCustomerSatellites_6264a7b0-419c-4a65-9a73-1ad23e65b90c" xlink:href="mnts-20230930.xsd#mnts_NumberOfCustomerSatellites"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfCustomerSatellites_6264a7b0-419c-4a65-9a73-1ad23e65b90c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfNewSpacecraftInOrbit_937ca95d-e8eb-40b2-8e03-3f617829fc8d" xlink:href="mnts-20230930.xsd#mnts_NumberOfNewSpacecraftInOrbit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfNewSpacecraftInOrbit_937ca95d-e8eb-40b2-8e03-3f617829fc8d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfSpacecraftMissions_fb7abd62-cfca-433e-b786-9df602766056" xlink:href="mnts-20230930.xsd#mnts_NumberOfSpacecraftMissions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfSpacecraftMissions_fb7abd62-cfca-433e-b786-9df602766056" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfCustomerSatellitesDeployedInOrbit_06d8ef28-57c4-4580-9d9d-2e1d837eb062" xlink:href="mnts-20230930.xsd#mnts_NumberOfCustomerSatellitesDeployedInOrbit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfCustomerSatellitesDeployedInOrbit_06d8ef28-57c4-4580-9d9d-2e1d837eb062" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfSatellitesToBeProducedUnderBid_690304c8-16fe-461f-8443-bbfd007e3f41" xlink:href="mnts-20230930.xsd#mnts_NumberOfSatellitesToBeProducedUnderBid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_NumberOfSatellitesToBeProducedUnderBid_690304c8-16fe-461f-8443-bbfd007e3f41" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationProceedsReceivedFromShareholders_4b4c784e-ac6f-4282-a685-d0edf5cc57b0" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationProceedsReceivedFromShareholders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_ReverseRecapitalizationProceedsReceivedFromShareholders_4b4c784e-ac6f-4282-a685-d0edf5cc57b0" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_d8fcd076-3963-463f-98e8-65b02e87ef7d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_NetIncomeLoss_d8fcd076-3963-463f-98e8-65b02e87ef7d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_5399f115-fd61-4fb8-95a0-4a1a1147f389" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_5399f115-fd61-4fb8-95a0-4a1a1147f389" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_0999c09b-5bd9-4c3f-a7ef-b36a8145604b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_0999c09b-5bd9-4c3f-a7ef-b36a8145604b" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e1ad9919-8f9a-425f-a9ab-78d032b1ee41" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e1ad9919-8f9a-425f-a9ab-78d032b1ee41" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_c2fd77e6-0e62-4861-b644-89ab66de5754" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_c2fd77e6-0e62-4861-b644-89ab66de5754" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_4030463a-67dd-47f5-8827-09de4bb58f23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_4030463a-67dd-47f5-8827-09de4bb58f23" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ScheduleOfReverseRecapitalizationTable_a8b5a975-63ed-40c8-b8de-6f7dc0dc55b5" xlink:href="mnts-20230930.xsd#mnts_ScheduleOfReverseRecapitalizationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationLineItems_a2cb57a6-7eb3-42f4-bfaa-396dea404bd7" xlink:to="loc_mnts_ScheduleOfReverseRecapitalizationTable_a8b5a975-63ed-40c8-b8de-6f7dc0dc55b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_c075ae28-c4cc-4bc3-a0d5-de3687a27e66" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationTable_a8b5a975-63ed-40c8-b8de-6f7dc0dc55b5" xlink:to="loc_srt_StatementScenarioAxis_c075ae28-c4cc-4bc3-a0d5-de3687a27e66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_c075ae28-c4cc-4bc3-a0d5-de3687a27e66_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_c075ae28-c4cc-4bc3-a0d5-de3687a27e66" xlink:to="loc_srt_ScenarioUnspecifiedDomain_c075ae28-c4cc-4bc3-a0d5-de3687a27e66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_7851527b-3a3e-405f-91c4-8d1bb3a86104" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_c075ae28-c4cc-4bc3-a0d5-de3687a27e66" xlink:to="loc_srt_ScenarioUnspecifiedDomain_7851527b-3a3e-405f-91c4-8d1bb3a86104" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_8bed250f-c4c4-4071-8bac-5bd793cdfc82" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_7851527b-3a3e-405f-91c4-8d1bb3a86104" xlink:to="loc_srt_ScenarioForecastMember_8bed250f-c4c4-4071-8bac-5bd793cdfc82" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1a00e700-c84c-4a49-afb7-e12bf7b73ff6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationTable_a8b5a975-63ed-40c8-b8de-6f7dc0dc55b5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1a00e700-c84c-4a49-afb7-e12bf7b73ff6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1a00e700-c84c-4a49-afb7-e12bf7b73ff6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1a00e700-c84c-4a49-afb7-e12bf7b73ff6" xlink:to="loc_us-gaap_ClassOfStockDomain_1a00e700-c84c-4a49-afb7-e12bf7b73ff6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_db0851dc-6d20-4ee4-97d8-4b2f85dc8b25" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1a00e700-c84c-4a49-afb7-e12bf7b73ff6" xlink:to="loc_us-gaap_ClassOfStockDomain_db0851dc-6d20-4ee4-97d8-4b2f85dc8b25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_5f7c50f1-e5b9-43a4-ba0b-9f10c6b4d960" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_db0851dc-6d20-4ee4-97d8-4b2f85dc8b25" xlink:to="loc_us-gaap_CommonClassAMember_5f7c50f1-e5b9-43a4-ba0b-9f10c6b4d960" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_5bb7477c-a69c-4019-abda-411bb58b0b22" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mnts_ScheduleOfReverseRecapitalizationTable_a8b5a975-63ed-40c8-b8de-6f7dc0dc55b5" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_5bb7477c-a69c-4019-abda-411bb58b0b22" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_5bb7477c-a69c-4019-abda-411bb58b0b22_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_5bb7477c-a69c-4019-abda-411bb58b0b22" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_5bb7477c-a69c-4019-abda-411bb58b0b22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_b2646063-dd1a-46a2-901e-c2b85a399bd1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_5bb7477c-a69c-4019-abda-411bb58b0b22" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_b2646063-dd1a-46a2-901e-c2b85a399bd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_535bef35-3947-44f6-bcb9-7565f82e10dc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_b2646063-dd1a-46a2-901e-c2b85a399bd1" xlink:to="loc_us-gaap_SubsequentEventMember_535bef35-3947-44f6-bcb9-7565f82e10dc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SummaryofSignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_9937b2cb-3eb2-4502-9767-7c0b9a18b697" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_9937b2cb-3eb2-4502-9767-7c0b9a18b697" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_e322eb69-25ca-4225-97c6-ba68a693a8ae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_RestrictedCash_e322eb69-25ca-4225-97c6-ba68a693a8ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_DeferredFulfillmentAndPrepaidLaunchCosts_6d750195-1308-4891-876a-53911c368477" xlink:href="mnts-20230930.xsd#mnts_DeferredFulfillmentAndPrepaidLaunchCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_DeferredFulfillmentAndPrepaidLaunchCosts_6d750195-1308-4891-876a-53911c368477" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_ff51d2a3-e6de-4ac9-8168-b08c84b6eaf9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_ff51d2a3-e6de-4ac9-8168-b08c84b6eaf9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_8efc91f6-8fe7-4e71-862d-205fad4d5f1a" xlink:href="mnts-20230930.xsd#mnts_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_8efc91f6-8fe7-4e71-862d-205fad4d5f1a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_aae03580-dd0d-4334-b07a-71ab823481e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_aae03580-dd0d-4334-b07a-71ab823481e0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_c45a248a-da36-4302-85d9-e2ffdabc2bd9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_c45a248a-da36-4302-85d9-e2ffdabc2bd9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_09863711-7f9f-4651-9583-de986c364fb6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_09863711-7f9f-4651-9583-de986c364fb6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SaleOfStockPaymentOfLiability_db58da62-9705-4abf-8545-2b4bc739d1b2" xlink:href="mnts-20230930.xsd#mnts_SaleOfStockPaymentOfLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_SaleOfStockPaymentOfLiability_db58da62-9705-4abf-8545-2b4bc739d1b2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_b24ac06e-286f-4eb5-95a2-ab619136b879" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_b24ac06e-286f-4eb5-95a2-ab619136b879" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_RecapitalizationExchangeRatio_a9d49301-f08b-45dd-b7d1-d8c4ae0aaba5" xlink:href="mnts-20230930.xsd#mnts_RecapitalizationExchangeRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_RecapitalizationExchangeRatio_a9d49301-f08b-45dd-b7d1-d8c4ae0aaba5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_52aaca92-6fb3-4d76-b120-e3d39060f90a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_52aaca92-6fb3-4d76-b120-e3d39060f90a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_825472cf-5f02-40f7-90f7-8509ae351614" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_825472cf-5f02-40f7-90f7-8509ae351614" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WarrantLiabilityNoncurrent_bcbe4566-e302-4fd6-bb52-f1488775dc61" xlink:href="mnts-20230930.xsd#mnts_WarrantLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_WarrantLiabilityNoncurrent_bcbe4566-e302-4fd6-bb52-f1488775dc61" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_40a4b524-3d55-4859-b1e2-437401c6e910" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ImpairmentOfIntangibleAssetsFinitelived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_40a4b524-3d55-4859-b1e2-437401c6e910" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_e71b791f-c3b1-4878-9338-45f53e0e68ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_e71b791f-c3b1-4878-9338-45f53e0e68ef" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockPlanOfferingPeriod_ad7d6102-95de-40b4-8a06-5148d83f7f38" xlink:href="mnts-20230930.xsd#mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockPlanOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockPlanOfferingPeriod_ad7d6102-95de-40b4-8a06-5148d83f7f38" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NumberOfOperatingServiceLines_6de97f6f-509e-404f-842a-ab4bfa902db1" xlink:href="mnts-20230930.xsd#mnts_NumberOfOperatingServiceLines"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_mnts_NumberOfOperatingServiceLines_6de97f6f-509e-404f-842a-ab4bfa902db1" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_290f504c-5ecf-4d61-8448-a39caef60511" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_NumberOfReportableSegments_290f504c-5ecf-4d61-8448-a39caef60511" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_e34f39d9-3538-463f-98cf-873a3c8c45fc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_NumberOfOperatingSegments_e34f39d9-3538-463f-98cf-873a3c8c45fc" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_606eaad6-19d7-4d91-8390-6a7b85c21f92" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_6e145de2-f412-4c6e-8ec4-95925daf6778" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_6e145de2-f412-4c6e-8ec4-95925daf6778" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_6e145de2-f412-4c6e-8ec4-95925daf6778_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_6e145de2-f412-4c6e-8ec4-95925daf6778" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_6e145de2-f412-4c6e-8ec4-95925daf6778_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_8261b861-1f47-45c5-9610-6de23608caf5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_6e145de2-f412-4c6e-8ec4-95925daf6778" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_8261b861-1f47-45c5-9610-6de23608caf5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_CashDesignatedForCollateralMember_0775e758-8462-4596-b9c0-1219839a6422" xlink:href="mnts-20230930.xsd#mnts_CashDesignatedForCollateralMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_8261b861-1f47-45c5-9610-6de23608caf5" xlink:to="loc_mnts_CashDesignatedForCollateralMember_0775e758-8462-4596-b9c0-1219839a6422" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_CashDesignatedForExpendituresUnderNationalSecurityAgreementMember_a244b27b-d84b-49c8-bd31-b63fbbe4f2f8" xlink:href="mnts-20230930.xsd#mnts_CashDesignatedForExpendituresUnderNationalSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_8261b861-1f47-45c5-9610-6de23608caf5" xlink:to="loc_mnts_CashDesignatedForExpendituresUnderNationalSecurityAgreementMember_a244b27b-d84b-49c8-bd31-b63fbbe4f2f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4a77f14b-a808-466d-882f-b8bb02097cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4a77f14b-a808-466d-882f-b8bb02097cc9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4a77f14b-a808-466d-882f-b8bb02097cc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4a77f14b-a808-466d-882f-b8bb02097cc9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4a77f14b-a808-466d-882f-b8bb02097cc9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4a03dc2f-c633-447d-91b5-665cda040221" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4a77f14b-a808-466d-882f-b8bb02097cc9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4a03dc2f-c633-447d-91b5-665cda040221" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PatentsMember_6f4370df-e4be-44f6-b697-5239bb318ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PatentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4a03dc2f-c633-447d-91b5-665cda040221" xlink:to="loc_us-gaap_PatentsMember_6f4370df-e4be-44f6-b697-5239bb318ac9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_9ed41d9b-4193-48c8-afeb-b2de00940bb4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_9ed41d9b-4193-48c8-afeb-b2de00940bb4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_9ed41d9b-4193-48c8-afeb-b2de00940bb4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_9ed41d9b-4193-48c8-afeb-b2de00940bb4" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_9ed41d9b-4193-48c8-afeb-b2de00940bb4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_34d0f3ee-edfd-4058-b245-c2939d3b1455" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_9ed41d9b-4193-48c8-afeb-b2de00940bb4" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_34d0f3ee-edfd-4058-b245-c2939d3b1455" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PrivatePlacementWarrantMember_6b90b009-bf2c-4f9e-a890-4f41892663ab" xlink:href="mnts-20230930.xsd#mnts_PrivatePlacementWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_34d0f3ee-edfd-4058-b245-c2939d3b1455" xlink:to="loc_mnts_PrivatePlacementWarrantMember_6b90b009-bf2c-4f9e-a890-4f41892663ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PublicWarrantMember_3fd9c0a4-44f7-47da-b66b-c96e14b1eac5" xlink:href="mnts-20230930.xsd#mnts_PublicWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_34d0f3ee-edfd-4058-b245-c2939d3b1455" xlink:to="loc_mnts_PublicWarrantMember_3fd9c0a4-44f7-47da-b66b-c96e14b1eac5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b6fd8470-314d-4f2c-b552-7d26f848cf68" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:to="loc_us-gaap_AwardTypeAxis_b6fd8470-314d-4f2c-b552-7d26f848cf68" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b6fd8470-314d-4f2c-b552-7d26f848cf68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b6fd8470-314d-4f2c-b552-7d26f848cf68" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b6fd8470-314d-4f2c-b552-7d26f848cf68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e9861bcd-b94a-4418-90a7-e415336ad9d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b6fd8470-314d-4f2c-b552-7d26f848cf68" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e9861bcd-b94a-4418-90a7-e415336ad9d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_3decfe35-4b95-4220-921a-2b6cc38c0bad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e9861bcd-b94a-4418-90a7-e415336ad9d7" xlink:to="loc_us-gaap_EmployeeStockMember_3decfe35-4b95-4220-921a-2b6cc38c0bad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_3aebfcbb-4029-434a-a010-af5a441e29d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e9861bcd-b94a-4418-90a7-e415336ad9d7" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_3aebfcbb-4029-434a-a010-af5a441e29d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_f6e532dd-21ba-4658-b73b-77745f79e372" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_feb91a4c-9be9-4dcf-a71d-732ef3cc1d7a" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_f6e532dd-21ba-4658-b73b-77745f79e372" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_f6e532dd-21ba-4658-b73b-77745f79e372_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_f6e532dd-21ba-4658-b73b-77745f79e372" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_f6e532dd-21ba-4658-b73b-77745f79e372_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1855d461-b194-470b-8969-5e2a8da809af" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_f6e532dd-21ba-4658-b73b-77745f79e372" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1855d461-b194-470b-8969-5e2a8da809af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_dd4941ec-db21-46a6-8cc0-2a2c92019725" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_1855d461-b194-470b-8969-5e2a8da809af" xlink:to="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_dd4941ec-db21-46a6-8cc0-2a2c92019725" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_dfd9b5b1-6556-48ff-bdf4-7bde10b34e63" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_1855d461-b194-470b-8969-5e2a8da809af" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_dfd9b5b1-6556-48ff-bdf4-7bde10b34e63" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofPropertyMachineryandEquipmentnetDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SummaryofSignificantAccountingPoliciesScheduleofPropertyMachineryandEquipmentnetDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofPropertyMachineryandEquipmentnetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0bcb78e9-ce98-4ac6-bb7d-35bbee6d4cdf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_37abb776-93c7-4287-b68c-e00992e94f68" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0bcb78e9-ce98-4ac6-bb7d-35bbee6d4cdf" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_37abb776-93c7-4287-b68c-e00992e94f68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_084b6816-b5a4-4893-9b0d-d90f69cdd619" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0bcb78e9-ce98-4ac6-bb7d-35bbee6d4cdf" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_084b6816-b5a4-4893-9b0d-d90f69cdd619" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83d6c218-eb97-469d-923b-8ff90b920b73" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_084b6816-b5a4-4893-9b0d-d90f69cdd619" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83d6c218-eb97-469d-923b-8ff90b920b73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_83d6c218-eb97-469d-923b-8ff90b920b73_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83d6c218-eb97-469d-923b-8ff90b920b73" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_83d6c218-eb97-469d-923b-8ff90b920b73_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83d6c218-eb97-469d-923b-8ff90b920b73" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_1bbfaa58-516f-4a5d-84c1-0ce746c980b7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:to="loc_us-gaap_ComputerEquipmentMember_1bbfaa58-516f-4a5d-84c1-0ce746c980b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_c97d0901-382e-4778-8d0b-74fc88ae5eaa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_c97d0901-382e-4778-8d0b-74fc88ae5eaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_b45be532-c8fb-41c8-8c6b-f465eb4f7cf9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_b45be532-c8fb-41c8-8c6b-f465eb4f7cf9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_967ac609-8554-4d17-a25c-68db784167da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8884d4af-fe85-4d92-b542-aae09a11ffe7" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_967ac609-8554-4d17-a25c-68db784167da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_5c248f8e-b6eb-4e65-8baf-fdcf8834f129" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_084b6816-b5a4-4893-9b0d-d90f69cdd619" xlink:to="loc_srt_RangeAxis_5c248f8e-b6eb-4e65-8baf-fdcf8834f129" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5c248f8e-b6eb-4e65-8baf-fdcf8834f129_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_5c248f8e-b6eb-4e65-8baf-fdcf8834f129" xlink:to="loc_srt_RangeMember_5c248f8e-b6eb-4e65-8baf-fdcf8834f129_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_e754e549-e78a-4afb-8773-e537b9a9adb4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_5c248f8e-b6eb-4e65-8baf-fdcf8834f129" xlink:to="loc_srt_RangeMember_e754e549-e78a-4afb-8773-e537b9a9adb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_2bf979b9-f9aa-43b5-9d39-962e708be1e2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_e754e549-e78a-4afb-8773-e537b9a9adb4" xlink:to="loc_srt_MinimumMember_2bf979b9-f9aa-43b5-9d39-962e708be1e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_ab3aab36-a812-4015-afd6-d23be31432ea" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_e754e549-e78a-4afb-8773-e537b9a9adb4" xlink:to="loc_srt_MaximumMember_ab3aab36-a812-4015-afd6-d23be31432ea" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_dde5331e-70c1-45df-9f2f-cf0f74104e2c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a02f9d2e-e9f2-487d-8a26-61cbb9713a8d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_dde5331e-70c1-45df-9f2f-cf0f74104e2c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a02f9d2e-e9f2-487d-8a26-61cbb9713a8d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_376d3df3-00a1-4510-ac3e-0948c2b8b28c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_dde5331e-70c1-45df-9f2f-cf0f74104e2c" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_376d3df3-00a1-4510-ac3e-0948c2b8b28c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_a3368c0b-4683-4898-bd4d-bc3bb1a5b630" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_376d3df3-00a1-4510-ac3e-0948c2b8b28c" xlink:to="loc_srt_ProductOrServiceAxis_a3368c0b-4683-4898-bd4d-bc3bb1a5b630" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_a3368c0b-4683-4898-bd4d-bc3bb1a5b630_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_a3368c0b-4683-4898-bd4d-bc3bb1a5b630" xlink:to="loc_srt_ProductsAndServicesDomain_a3368c0b-4683-4898-bd4d-bc3bb1a5b630_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_81a02ed3-7496-4f1b-9c5f-74fb716c58b6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_a3368c0b-4683-4898-bd4d-bc3bb1a5b630" xlink:to="loc_srt_ProductsAndServicesDomain_81a02ed3-7496-4f1b-9c5f-74fb716c58b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_TransportationServicesMember_2ef6f044-29c9-44b6-926a-9792f63f8476" xlink:href="mnts-20230930.xsd#mnts_TransportationServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_81a02ed3-7496-4f1b-9c5f-74fb716c58b6" xlink:to="loc_mnts_TransportationServicesMember_2ef6f044-29c9-44b6-926a-9792f63f8476" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_HostedPayloadServicesMember_51a951fa-e6c6-4e4b-b558-d337ae6b62bd" xlink:href="mnts-20230930.xsd#mnts_HostedPayloadServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_81a02ed3-7496-4f1b-9c5f-74fb716c58b6" xlink:to="loc_mnts_HostedPayloadServicesMember_51a951fa-e6c6-4e4b-b558-d337ae6b62bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ForfeitedCustomerDepositsMember_81dd0f72-7984-41fc-a56a-f06492d11b4c" xlink:href="mnts-20230930.xsd#mnts_ForfeitedCustomerDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_81a02ed3-7496-4f1b-9c5f-74fb716c58b6" xlink:to="loc_mnts_ForfeitedCustomerDepositsMember_81dd0f72-7984-41fc-a56a-f06492d11b4c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofChangesinFairValueofLiabilitiesDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SummaryofSignificantAccountingPoliciesScheduleofChangesinFairValueofLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofChangesinFairValueofLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_9f5757d3-642a-4721-84fc-f213c9cb7c1e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_8938f7bf-e41d-47da-b0a6-321d56091b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_9f5757d3-642a-4721-84fc-f213c9cb7c1e" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_8938f7bf-e41d-47da-b0a6-321d56091b2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_479aac88-9100-4add-b1fe-63906f4053c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_8938f7bf-e41d-47da-b0a6-321d56091b2e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_479aac88-9100-4add-b1fe-63906f4053c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_fcbcf983-e6d7-48c8-ad3d-bf34d6bbea3e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_8938f7bf-e41d-47da-b0a6-321d56091b2e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_fcbcf983-e6d7-48c8-ad3d-bf34d6bbea3e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome_418533c0-c4c7-48fb-b13c-d125eaa374c3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_8938f7bf-e41d-47da-b0a6-321d56091b2e" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome_418533c0-c4c7-48fb-b13c-d125eaa374c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_3b9ad716-658d-4adf-b7b2-8f7dd366ffbf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_416acbe3-3d8f-4c7c-b7e9-c1b012301c5d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_9f5757d3-642a-4721-84fc-f213c9cb7c1e" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_416acbe3-3d8f-4c7c-b7e9-c1b012301c5d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e0851221-eb13-4a8f-9440-2e6173397d55" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_416acbe3-3d8f-4c7c-b7e9-c1b012301c5d" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e0851221-eb13-4a8f-9440-2e6173397d55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_e0851221-eb13-4a8f-9440-2e6173397d55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e0851221-eb13-4a8f-9440-2e6173397d55" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_e0851221-eb13-4a8f-9440-2e6173397d55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f7b2e6a2-75bf-43bb-b854-5a42630c6bca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e0851221-eb13-4a8f-9440-2e6173397d55" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f7b2e6a2-75bf-43bb-b854-5a42630c6bca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_e7d29b81-ab4c-4b39-86b1-40cb778867d1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f7b2e6a2-75bf-43bb-b854-5a42630c6bca" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_e7d29b81-ab4c-4b39-86b1-40cb778867d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_d8be227e-5a89-4531-a08d-d7b980feb7c4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_416acbe3-3d8f-4c7c-b7e9-c1b012301c5d" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_d8be227e-5a89-4531-a08d-d7b980feb7c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_d8be227e-5a89-4531-a08d-d7b980feb7c4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_d8be227e-5a89-4531-a08d-d7b980feb7c4" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_d8be227e-5a89-4531-a08d-d7b980feb7c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_0e66fd68-06b8-46c4-bd47-34e361e18b6c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_d8be227e-5a89-4531-a08d-d7b980feb7c4" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_0e66fd68-06b8-46c4-bd47-34e361e18b6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_84293a48-e5ea-46d3-8e2e-fb379498a9e2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_0e66fd68-06b8-46c4-bd47-34e361e18b6c" xlink:to="loc_us-gaap_WarrantMember_84293a48-e5ea-46d3-8e2e-fb379498a9e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareRepurchaseLiabilityMember_8ec646b0-0bf2-4d33-9b6d-6e43d62ef558" xlink:href="mnts-20230930.xsd#mnts_ShareRepurchaseLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_0e66fd68-06b8-46c4-bd47-34e361e18b6c" xlink:to="loc_mnts_ShareRepurchaseLiabilityMember_8ec646b0-0bf2-4d33-9b6d-6e43d62ef558" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2a5ea88b-78c7-4ff6-8c59-716401fbba21" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_416acbe3-3d8f-4c7c-b7e9-c1b012301c5d" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2a5ea88b-78c7-4ff6-8c59-716401fbba21" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_2a5ea88b-78c7-4ff6-8c59-716401fbba21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2a5ea88b-78c7-4ff6-8c59-716401fbba21" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_2a5ea88b-78c7-4ff6-8c59-716401fbba21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_d0cce35f-c820-4ec7-9148-6121cc907deb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2a5ea88b-78c7-4ff6-8c59-716401fbba21" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_d0cce35f-c820-4ec7-9148-6121cc907deb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_3098efd0-d6c4-4536-8d06-26b23b572c37" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_d0cce35f-c820-4ec7-9148-6121cc907deb" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_3098efd0-d6c4-4536-8d06-26b23b572c37" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofFairValueInputsDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SummaryofSignificantAccountingPoliciesScheduleofFairValueInputsDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SummaryofSignificantAccountingPoliciesScheduleofFairValueInputsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_5b9d4153-5572-494e-94de-b2e09562fb12" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_1ad8add4-2662-4a0a-b6a7-2995a49e2ae3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_5b9d4153-5572-494e-94de-b2e09562fb12" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_1ad8add4-2662-4a0a-b6a7-2995a49e2ae3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_713cbcdb-ca82-4826-b2d5-f3ade03e52e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_5b9d4153-5572-494e-94de-b2e09562fb12" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_713cbcdb-ca82-4826-b2d5-f3ade03e52e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_751e8b7f-2a5b-45e3-8dce-faf15a2ad73c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_5b9d4153-5572-494e-94de-b2e09562fb12" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_751e8b7f-2a5b-45e3-8dce-faf15a2ad73c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_96e19928-874e-4e1e-98d8-4c528bb612d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_751e8b7f-2a5b-45e3-8dce-faf15a2ad73c" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_96e19928-874e-4e1e-98d8-4c528bb612d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_96e19928-874e-4e1e-98d8-4c528bb612d7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_96e19928-874e-4e1e-98d8-4c528bb612d7" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_96e19928-874e-4e1e-98d8-4c528bb612d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_96e19928-874e-4e1e-98d8-4c528bb612d7" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_79594662-4ae8-4850-bcb6-f05f3a00242b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_79594662-4ae8-4850-bcb6-f05f3a00242b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputOptionVolatilityMember_872dd0ba-d7b1-4dbb-80cc-e06d4a51b8a2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputOptionVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:to="loc_us-gaap_MeasurementInputOptionVolatilityMember_872dd0ba-d7b1-4dbb-80cc-e06d4a51b8a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_9496407d-0ef7-4ded-97c1-e9a1274b0ad2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_9496407d-0ef7-4ded-97c1-e9a1274b0ad2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_b86dd2a5-7745-4ef8-a34c-e4dbbcd6e8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_8e6828ed-eefd-4fd8-9935-c076778cad08" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_b86dd2a5-7745-4ef8-a34c-e4dbbcd6e8ee" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/PropertyMachineryandEquipmentnetDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#PropertyMachineryandEquipmentnetDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/PropertyMachineryandEquipmentnetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_fe49bd42-d3b3-49a4-9df2-7e62c525a7d6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_fe49bd42-d3b3-49a4-9df2-7e62c525a7d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_e4297317-db6d-4e01-a271-5f3fe20f5d01" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_e4297317-db6d-4e01-a271-5f3fe20f5d01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_7fb69ee0-2e1e-426c-9c86-d0a6d5b205da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_7fb69ee0-2e1e-426c-9c86-d0a6d5b205da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_f6a08c36-6841-4ad3-9ffe-a33b817ccf16" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:to="loc_us-gaap_Depreciation_f6a08c36-6841-4ad3-9ffe-a33b817ccf16" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_f7a5f30b-32ab-4eb3-a697-a096a98880bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_65d18db6-2b34-43ef-9dff-db8c89ea1495" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_f7a5f30b-32ab-4eb3-a697-a096a98880bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_7e03b6b6-e490-428a-aa40-296d0e128dc7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_f7a5f30b-32ab-4eb3-a697-a096a98880bc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_7e03b6b6-e490-428a-aa40-296d0e128dc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_7e03b6b6-e490-428a-aa40-296d0e128dc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_7e03b6b6-e490-428a-aa40-296d0e128dc7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_7e03b6b6-e490-428a-aa40-296d0e128dc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_7e03b6b6-e490-428a-aa40-296d0e128dc7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_7d97b898-7c98-4b50-a1d1-bd42a2fc670b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:to="loc_us-gaap_ComputerEquipmentMember_7d97b898-7c98-4b50-a1d1-bd42a2fc670b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_176f0a72-e4ba-4263-a60a-271fe9e42dc2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_176f0a72-e4ba-4263-a60a-271fe9e42dc2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_5b645437-09f0-4094-a3a7-5306ef3e584c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_5b645437-09f0-4094-a3a7-5306ef3e584c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_4c04c717-eaa9-4798-8c0d-7f391c540a19" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_218754e7-b226-45d0-b5cf-c34df8915393" xlink:to="loc_us-gaap_ConstructionInProgressMember_4c04c717-eaa9-4798-8c0d-7f391c540a19" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/IntangibleAssetsnetScheduleofIntangibleAssetsNetDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#IntangibleAssetsnetScheduleofIntangibleAssetsNetDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/IntangibleAssetsnetScheduleofIntangibleAssetsNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_22ab29ae-e272-4919-8a5a-26b6bb7fe72d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_22ab29ae-e272-4919-8a5a-26b6bb7fe72d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_436acd95-7e33-4015-b8d5-7c173c1112b5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_436acd95-7e33-4015-b8d5-7c173c1112b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a360e267-762b-4a34-b944-5956963e681a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a360e267-762b-4a34-b944-5956963e681a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_0c65b182-76cb-4737-a031-76cd170d611c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_0c65b182-76cb-4737-a031-76cd170d611c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_93dc4bf3-0431-476d-b016-c337db04de54" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a7e970b3-eef2-4f4e-983b-19ee813554e7" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_93dc4bf3-0431-476d-b016-c337db04de54" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d3443ff8-1fb3-44d2-9366-b2b2540aeede" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_93dc4bf3-0431-476d-b016-c337db04de54" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d3443ff8-1fb3-44d2-9366-b2b2540aeede" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d3443ff8-1fb3-44d2-9366-b2b2540aeede_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d3443ff8-1fb3-44d2-9366-b2b2540aeede" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d3443ff8-1fb3-44d2-9366-b2b2540aeede_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31b1406e-1673-4cd1-8cc0-b039be7d2a3d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d3443ff8-1fb3-44d2-9366-b2b2540aeede" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31b1406e-1673-4cd1-8cc0-b039be7d2a3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntellectualPropertyMember_292ad3c4-a037-412a-9143-c8ad95fbcf03" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntellectualPropertyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_31b1406e-1673-4cd1-8cc0-b039be7d2a3d" xlink:to="loc_us-gaap_IntellectualPropertyMember_292ad3c4-a037-412a-9143-c8ad95fbcf03" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/LoanPayableDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#LoanPayableDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/LoanPayableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_c8f0513e-0017-402d-9aad-d4044f9ec0ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_c8f0513e-0017-402d-9aad-d4044f9ec0ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9d429cf1-00ae-4149-bbdf-3e1ab6550101" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9d429cf1-00ae-4149-bbdf-3e1ab6550101" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromShortTermDebt_39697f3d-ccf5-41fa-a25e-d7b7d073d747" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_ProceedsFromShortTermDebt_39697f3d-ccf5-41fa-a25e-d7b7d073d747" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_DebtInstrumentReductionInFaceAmount_97d1deba-3620-4413-a28f-b68733a0a1bc" xlink:href="mnts-20230930.xsd#mnts_DebtInstrumentReductionInFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_mnts_DebtInstrumentReductionInFaceAmount_97d1deba-3620-4413-a28f-b68733a0a1bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_bcfc2c1b-0504-4832-bd8f-8d7f760f66d8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentTerm_bcfc2c1b-0504-4832-bd8f-8d7f760f66d8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet_17418b59-4885-4632-ad37-08aee9900fba" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet_17418b59-4885-4632-ad37-08aee9900fba" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_DebtInstrumentDiscountRateInExcessOfInitialFaceValuePercentage_b62b0478-593e-4a16-aabf-7d5e81a1a175" xlink:href="mnts-20230930.xsd#mnts_DebtInstrumentDiscountRateInExcessOfInitialFaceValuePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_mnts_DebtInstrumentDiscountRateInExcessOfInitialFaceValuePercentage_b62b0478-593e-4a16-aabf-7d5e81a1a175" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_74a8b992-1efd-4362-bad2-34645cb59ac0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_74a8b992-1efd-4362-bad2-34645cb59ac0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_a23327cd-8467-42f2-8482-0c2a66b45a81" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_InterestExpenseDebt_a23327cd-8467-42f2-8482-0c2a66b45a81" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansPayable_d6286626-f596-458a-84bf-22b458d577b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LoansPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_LoansPayable_d6286626-f596-458a-84bf-22b458d577b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrentAndNoncurrent_f4e4f2ba-3ab9-4714-9fa5-6fc0b71da2bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_InterestPayableCurrentAndNoncurrent_f4e4f2ba-3ab9-4714-9fa5-6fc0b71da2bc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_248a76fd-0ed1-45aa-98ad-a8e9f27b95d2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_248a76fd-0ed1-45aa-98ad-a8e9f27b95d2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_b54494dd-22b8-4f05-a350-ca844cd9782e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_b54494dd-22b8-4f05-a350-ca844cd9782e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_1cc91821-c11f-4592-861f-3f92d1fccc97" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_1cc91821-c11f-4592-861f-3f92d1fccc97" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShortTermDebtTable_da960843-5255-40d9-99cf-9764d3df43d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShortTermDebtTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShortTermDebtLineItems_f304b138-50bf-4681-85f7-6add30bace2d" xlink:to="loc_us-gaap_ScheduleOfShortTermDebtTable_da960843-5255-40d9-99cf-9764d3df43d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_da960843-5255-40d9-99cf-9764d3df43d7" xlink:to="loc_us-gaap_DebtInstrumentAxis_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9d68ddcf-bef9-48bc-991d-fd0d8917aa03" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2a3767c7-be3d-4f1b-94a4-a3842d7bf68a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9d68ddcf-bef9-48bc-991d-fd0d8917aa03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_TermLoanMember_26af0aca-5e94-4077-a425-474c6fb3e40f" xlink:href="mnts-20230930.xsd#mnts_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9d68ddcf-bef9-48bc-991d-fd0d8917aa03" xlink:to="loc_mnts_TermLoanMember_26af0aca-5e94-4077-a425-474c6fb3e40f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/StockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#StockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_d751efbc-cd5c-43c3-b5df-38d5893e22e9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_d751efbc-cd5c-43c3-b5df-38d5893e22e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_CommonAndPreferredStockSharesAuthorized_22724665-4664-4569-871f-f6361fc71259" xlink:href="mnts-20230930.xsd#mnts_CommonAndPreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_CommonAndPreferredStockSharesAuthorized_22724665-4664-4569-871f-f6361fc71259" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_28f9d152-2a49-43b8-bb05-7df7a41a033b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_28f9d152-2a49-43b8-bb05-7df7a41a033b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_68dcd45d-5b2f-4af4-94d4-0e7dd51dd613" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_68dcd45d-5b2f-4af4-94d4-0e7dd51dd613" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_9627e3e8-91fb-4a67-85c2-90d516dcc6e2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_9627e3e8-91fb-4a67-85c2-90d516dcc6e2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_24edfac4-b8c6-4c26-ad61-572fa8bb3aa0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_24edfac4-b8c6-4c26-ad61-572fa8bb3aa0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_e71db724-3560-4b83-9443-29ef6c4bb90e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_e71db724-3560-4b83-9443-29ef6c4bb90e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_5096d5d0-028a-4366-ac3f-306ff79a01b2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_5096d5d0-028a-4366-ac3f-306ff79a01b2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_c08237bc-4618-4aba-a5a1-c827871e5a0b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_c08237bc-4618-4aba-a5a1-c827871e5a0b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_c9abfb30-492d-4af4-b10a-dbd8bd794d80" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_c9abfb30-492d-4af4-b10a-dbd8bd794d80" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_35ae3a34-a9da-4b71-bb10-b72b8959eb92" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_35ae3a34-a9da-4b71-bb10-b72b8959eb92" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_e6e62ec2-87c4-487f-8ecd-3b4d190002f3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_e6e62ec2-87c4-487f-8ecd-3b4d190002f3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_94463f10-736c-4253-b801-f4e4b53ce5ca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_94463f10-736c-4253-b801-f4e4b53ce5ca" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired_bae56f79-4db1-4e6f-bee2-5dbfed0e352c" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired_bae56f79-4db1-4e6f-bee2-5dbfed0e352c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5a6597d5-6cbe-4bff-bb09-44a0d8c9a63f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5a6597d5-6cbe-4bff-bb09-44a0d8c9a63f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum_214e2022-5244-451b-989b-47391bcbe0fa" xlink:href="mnts-20230930.xsd#mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum_214e2022-5244-451b-989b-47391bcbe0fa" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationProceedsReceivedFromShareholders_5e394576-cb2d-4868-b9c5-b3eaf32002fc" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationProceedsReceivedFromShareholders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ReverseRecapitalizationProceedsReceivedFromShareholders_5e394576-cb2d-4868-b9c5-b3eaf32002fc" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SaleOfStockPaymentOfLiability_e2886245-a094-4db4-bfc5-ed8b8811c67b" xlink:href="mnts-20230930.xsd#mnts_SaleOfStockPaymentOfLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_SaleOfStockPaymentOfLiability_e2886245-a094-4db4-bfc5-ed8b8811c67b" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_a8f012eb-b6e1-4bd5-b6dc-4ce204822af8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_a8f012eb-b6e1-4bd5-b6dc-4ce204822af8" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAdjustmentOfWarrants_4d4421b0-820b-4a6a-aadf-ccd0a32ac7ea" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAdjustmentOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_FairValueAdjustmentOfWarrants_4d4421b0-820b-4a6a-aadf-ccd0a32ac7ea" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedPaymentArrangementNumberOfSharesModified_79dfdebc-7871-4ab8-ace6-46bfbcf539b5" xlink:href="mnts-20230930.xsd#mnts_ShareBasedPaymentArrangementNumberOfSharesModified"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ShareBasedPaymentArrangementNumberOfSharesModified_79dfdebc-7871-4ab8-ace6-46bfbcf539b5" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedPaymentArrangementWeightedAverageClosingPrice_0036087c-c106-427d-83b9-8f7e1ac2b3c9" xlink:href="mnts-20230930.xsd#mnts_ShareBasedPaymentArrangementWeightedAverageClosingPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ShareBasedPaymentArrangementWeightedAverageClosingPrice_0036087c-c106-427d-83b9-8f7e1ac2b3c9" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesForfeitedPercentage_c61fbe7d-6f80-425e-a56d-34eab01d9471" xlink:href="mnts-20230930.xsd#mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesForfeitedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesForfeitedPercentage_c61fbe7d-6f80-425e-a56d-34eab01d9471" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SaleOfStockCompensationAtCommissionRatePercentage_23125b08-9b3d-4838-bd03-62de8b1bf290" xlink:href="mnts-20230930.xsd#mnts_SaleOfStockCompensationAtCommissionRatePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_mnts_SaleOfStockCompensationAtCommissionRatePercentage_23125b08-9b3d-4838-bd03-62de8b1bf290" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_819b865e-3ee9-4ac6-b481-b33a7cdf5f61" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_4a393774-0fa2-433f-923c-ecd4c9286b64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_StatementClassOfStockAxis_4a393774-0fa2-433f-923c-ecd4c9286b64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4a393774-0fa2-433f-923c-ecd4c9286b64_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4a393774-0fa2-433f-923c-ecd4c9286b64" xlink:to="loc_us-gaap_ClassOfStockDomain_4a393774-0fa2-433f-923c-ecd4c9286b64_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_33ee83b2-dfb9-4aea-b6f2-f8c4f268c1e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4a393774-0fa2-433f-923c-ecd4c9286b64" xlink:to="loc_us-gaap_ClassOfStockDomain_33ee83b2-dfb9-4aea-b6f2-f8c4f268c1e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_6f506968-56e1-487f-8b31-1b25cfa3cc47" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_33ee83b2-dfb9-4aea-b6f2-f8c4f268c1e8" xlink:to="loc_us-gaap_CommonClassAMember_6f506968-56e1-487f-8b31-1b25cfa3cc47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_52fd7478-40e6-453d-a1ce-abd41a0db734" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_33ee83b2-dfb9-4aea-b6f2-f8c4f268c1e8" xlink:to="loc_us-gaap_PreferredStockMember_52fd7478-40e6-453d-a1ce-abd41a0db734" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_861cb435-e43f-4b8a-97af-e8b993cd9d9c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_861cb435-e43f-4b8a-97af-e8b993cd9d9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_861cb435-e43f-4b8a-97af-e8b993cd9d9c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_861cb435-e43f-4b8a-97af-e8b993cd9d9c" xlink:to="loc_us-gaap_EquityComponentDomain_861cb435-e43f-4b8a-97af-e8b993cd9d9c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_f990754b-833a-465c-ad61-547edd2f583a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_861cb435-e43f-4b8a-97af-e8b993cd9d9c" xlink:to="loc_us-gaap_EquityComponentDomain_f990754b-833a-465c-ad61-547edd2f583a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_5c972424-c999-408b-aad6-a102aa7261b9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f990754b-833a-465c-ad61-547edd2f583a" xlink:to="loc_us-gaap_CommonStockMember_5c972424-c999-408b-aad6-a102aa7261b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_331ef538-93bb-4066-b37c-a5c9d1c9afea" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f990754b-833a-465c-ad61-547edd2f583a" xlink:to="loc_us-gaap_WarrantMember_331ef538-93bb-4066-b37c-a5c9d1c9afea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_3249de4b-04a5-46bf-b60d-caa2317be074" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_3249de4b-04a5-46bf-b60d-caa2317be074" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_3249de4b-04a5-46bf-b60d-caa2317be074_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3249de4b-04a5-46bf-b60d-caa2317be074" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_3249de4b-04a5-46bf-b60d-caa2317be074_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3249de4b-04a5-46bf-b60d-caa2317be074" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PrefundedWarrantsMember_aeed32f4-e63c-47f1-b272-1ac35d41a062" xlink:href="mnts-20230930.xsd#mnts_PrefundedWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_PrefundedWarrantsMember_aeed32f4-e63c-47f1-b272-1ac35d41a062" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesAWarrantsMember_7188e7bd-67a0-4bee-bf14-8cd254451391" xlink:href="mnts-20230930.xsd#mnts_SeriesAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_SeriesAWarrantsMember_7188e7bd-67a0-4bee-bf14-8cd254451391" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesBWarrantsMember_d51fd7c4-8b39-4170-97d5-98c36b63bc71" xlink:href="mnts-20230930.xsd#mnts_SeriesBWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_SeriesBWarrantsMember_d51fd7c4-8b39-4170-97d5-98c36b63bc71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_FebruaryClassAWarrantsMember_522beed5-f687-4782-8917-fff283660d3c" xlink:href="mnts-20230930.xsd#mnts_FebruaryClassAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_FebruaryClassAWarrantsMember_522beed5-f687-4782-8917-fff283660d3c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PublicWarrantMember_13d47945-ae4d-4b76-a86d-a043b72ee628" xlink:href="mnts-20230930.xsd#mnts_PublicWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_PublicWarrantMember_13d47945-ae4d-4b76-a86d-a043b72ee628" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PrivatePlacementWarrantMember_4f9e312b-f82a-4bc2-808a-22bac9711f27" xlink:href="mnts-20230930.xsd#mnts_PrivatePlacementWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_PrivatePlacementWarrantMember_4f9e312b-f82a-4bc2-808a-22bac9711f27" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_OtherWarrantsMember_0268be65-2f10-464e-8422-db1c1b6cc9cf" xlink:href="mnts-20230930.xsd#mnts_OtherWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_OtherWarrantsMember_0268be65-2f10-464e-8422-db1c1b6cc9cf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PrivateWarrantMember_78e88015-722f-46f1-8152-9f4bfaedbb8f" xlink:href="mnts-20230930.xsd#mnts_PrivateWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e1783abf-4265-484e-958d-a0ff15475945" xlink:to="loc_mnts_PrivateWarrantMember_78e88015-722f-46f1-8152-9f4bfaedbb8f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_39bc823e-2841-44c4-9b8f-7f6788cc8383" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_39bc823e-2841-44c4-9b8f-7f6788cc8383" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_39bc823e-2841-44c4-9b8f-7f6788cc8383_default" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_39bc823e-2841-44c4-9b8f-7f6788cc8383" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_39bc823e-2841-44c4-9b8f-7f6788cc8383_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_91a44e58-1650-473e-8cfa-858face17540" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_39bc823e-2841-44c4-9b8f-7f6788cc8383" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_91a44e58-1650-473e-8cfa-858face17540" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ConsiderationFromFundsLegallyAvailableMember_f3c8c145-8401-47f6-8aa3-661c98b2f62f" xlink:href="mnts-20230930.xsd#mnts_ConsiderationFromFundsLegallyAvailableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_91a44e58-1650-473e-8cfa-858face17540" xlink:to="loc_mnts_ConsiderationFromFundsLegallyAvailableMember_f3c8c145-8401-47f6-8aa3-661c98b2f62f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e3fb4d23-d0e9-450b-a66b-71d287b4d263" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_AwardTypeAxis_e3fb4d23-d0e9-450b-a66b-71d287b4d263" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3fb4d23-d0e9-450b-a66b-71d287b4d263_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e3fb4d23-d0e9-450b-a66b-71d287b4d263" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3fb4d23-d0e9-450b-a66b-71d287b4d263_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5780c70a-b47e-4f51-92ec-646f04644a78" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e3fb4d23-d0e9-450b-a66b-71d287b4d263" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5780c70a-b47e-4f51-92ec-646f04644a78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ContingentSponsorEarnoutSharesMember_60a2a346-e6be-4f66-a3b8-e71d90f2b0da" xlink:href="mnts-20230930.xsd#mnts_ContingentSponsorEarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5780c70a-b47e-4f51-92ec-646f04644a78" xlink:to="loc_mnts_ContingentSponsorEarnoutSharesMember_60a2a346-e6be-4f66-a3b8-e71d90f2b0da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceAxis_5a52af10-1cb8-45d0-9d45-175509656d4c" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_mnts_WeightedAverageClosingPriceAxis_5a52af10-1cb8-45d0-9d45-175509656d4c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceDomain_5a52af10-1cb8-45d0-9d45-175509656d4c_default" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mnts_WeightedAverageClosingPriceAxis_5a52af10-1cb8-45d0-9d45-175509656d4c" xlink:to="loc_mnts_WeightedAverageClosingPriceDomain_5a52af10-1cb8-45d0-9d45-175509656d4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceDomain_b705ed46-899f-42fd-aa4f-e78b0b2811c3" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mnts_WeightedAverageClosingPriceAxis_5a52af10-1cb8-45d0-9d45-175509656d4c" xlink:to="loc_mnts_WeightedAverageClosingPriceDomain_b705ed46-899f-42fd-aa4f-e78b0b2811c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceTrancheOneMember_a7671c9a-c661-4167-b416-9aa31550f3fa" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_WeightedAverageClosingPriceDomain_b705ed46-899f-42fd-aa4f-e78b0b2811c3" xlink:to="loc_mnts_WeightedAverageClosingPriceTrancheOneMember_a7671c9a-c661-4167-b416-9aa31550f3fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceTrancheTwoMember_ad59ae45-be4a-4c06-bc3c-c50d310c5e56" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_WeightedAverageClosingPriceDomain_b705ed46-899f-42fd-aa4f-e78b0b2811c3" xlink:to="loc_mnts_WeightedAverageClosingPriceTrancheTwoMember_ad59ae45-be4a-4c06-bc3c-c50d310c5e56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_WeightedAverageClosingPriceTrancheThreeMember_cd7ebc8c-2769-45b7-ade2-19cdbd66871c" xlink:href="mnts-20230930.xsd#mnts_WeightedAverageClosingPriceTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_WeightedAverageClosingPriceDomain_b705ed46-899f-42fd-aa4f-e78b0b2811c3" xlink:to="loc_mnts_WeightedAverageClosingPriceTrancheThreeMember_cd7ebc8c-2769-45b7-ade2-19cdbd66871c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_d2e3be96-fb2d-487f-baa1-bd78fccc2c21" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_d2e3be96-fb2d-487f-baa1-bd78fccc2c21" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d2e3be96-fb2d-487f-baa1-bd78fccc2c21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d2e3be96-fb2d-487f-baa1-bd78fccc2c21" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d2e3be96-fb2d-487f-baa1-bd78fccc2c21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_86623e30-f398-4133-b87f-c0c975ccb3d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d2e3be96-fb2d-487f-baa1-bd78fccc2c21" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_86623e30-f398-4133-b87f-c0c975ccb3d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ATMSalesAgreementMember_a7b7a173-6180-42f3-ac67-b4eb593e53de" xlink:href="mnts-20230930.xsd#mnts_ATMSalesAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_86623e30-f398-4133-b87f-c0c975ccb3d0" xlink:to="loc_mnts_ATMSalesAgreementMember_a7b7a173-6180-42f3-ac67-b4eb593e53de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_a7a60cdc-f384-4c1f-a771-e5c7b5ade855" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f8aa96b3-141d-4ed2-8549-624ee158e333" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_a7a60cdc-f384-4c1f-a771-e5c7b5ade855" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_a7a60cdc-f384-4c1f-a771-e5c7b5ade855_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_a7a60cdc-f384-4c1f-a771-e5c7b5ade855" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_a7a60cdc-f384-4c1f-a771-e5c7b5ade855_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_406279b6-37fa-4bb4-9feb-87721a449e34" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_a7a60cdc-f384-4c1f-a771-e5c7b5ade855" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_406279b6-37fa-4bb4-9feb-87721a449e34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_3d7d93e1-5169-4664-974d-733bc26c1cdd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_406279b6-37fa-4bb4-9feb-87721a449e34" xlink:to="loc_us-gaap_SubsequentEventMember_3d7d93e1-5169-4664-974d-733bc26c1cdd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/StockholdersEquityScheduleofFairValueInputsDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#StockholdersEquityScheduleofFairValueInputsDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/StockholdersEquityScheduleofFairValueInputsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightLineItems_c27caad1-3fdc-4dca-9f0a-79d1719ae50f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_7d8fbf3e-f38b-434e-bc34-e9ddc7c76469" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_c27caad1-3fdc-4dca-9f0a-79d1719ae50f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_7d8fbf3e-f38b-434e-bc34-e9ddc7c76469" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_7e60146e-29e1-40fc-9fe4-eebf38ceec71" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_c27caad1-3fdc-4dca-9f0a-79d1719ae50f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_7e60146e-29e1-40fc-9fe4-eebf38ceec71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightTable_fabef8fc-de06-4c4c-8ed8-339c1747e88b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_c27caad1-3fdc-4dca-9f0a-79d1719ae50f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightTable_fabef8fc-de06-4c4c-8ed8-339c1747e88b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_fabef8fc-de06-4c4c-8ed8-339c1747e88b" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_86e90f83-202b-4c5f-ade2-e7abe83f8c80" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_afb12ccb-b948-4ef9-840c-c0b7e7cf51e2" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_86e90f83-202b-4c5f-ade2-e7abe83f8c80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesAWarrantsMember_339670dc-2316-40e2-99a8-738b2d9a4390" xlink:href="mnts-20230930.xsd#mnts_SeriesAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_86e90f83-202b-4c5f-ade2-e7abe83f8c80" xlink:to="loc_mnts_SeriesAWarrantsMember_339670dc-2316-40e2-99a8-738b2d9a4390" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesBWarrantsMember_da21aec3-8461-4efb-8456-ff5911865855" xlink:href="mnts-20230930.xsd#mnts_SeriesBWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_86e90f83-202b-4c5f-ade2-e7abe83f8c80" xlink:to="loc_mnts_SeriesBWarrantsMember_da21aec3-8461-4efb-8456-ff5911865855" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_FebruaryClassAWarrantsMember_52d456f0-d942-4bf8-9f93-d691dbf37943" xlink:href="mnts-20230930.xsd#mnts_FebruaryClassAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_86e90f83-202b-4c5f-ade2-e7abe83f8c80" xlink:to="loc_mnts_FebruaryClassAWarrantsMember_52d456f0-d942-4bf8-9f93-d691dbf37943" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_307d6e98-f494-4d51-8972-d9d6a573493d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_fabef8fc-de06-4c4c-8ed8-339c1747e88b" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_307d6e98-f494-4d51-8972-d9d6a573493d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_307d6e98-f494-4d51-8972-d9d6a573493d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_307d6e98-f494-4d51-8972-d9d6a573493d" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_307d6e98-f494-4d51-8972-d9d6a573493d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_dd4c6026-8983-4b64-ada0-bf5a920c0fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_307d6e98-f494-4d51-8972-d9d6a573493d" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_dd4c6026-8983-4b64-ada0-bf5a920c0fc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_c6d987f0-4d80-4b60-a6e1-fdcf5a095a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_dd4c6026-8983-4b64-ada0-bf5a920c0fc9" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_c6d987f0-4d80-4b60-a6e1-fdcf5a095a6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_7559a8a8-e900-47b6-acaf-09f913b27110" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_dd4c6026-8983-4b64-ada0-bf5a920c0fc9" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_7559a8a8-e900-47b6-acaf-09f913b27110" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_64bd8568-ef02-452d-9443-8439702db683" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_dd4c6026-8983-4b64-ada0-bf5a920c0fc9" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_64bd8568-ef02-452d-9443-8439702db683" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/StockbasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#StockbasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/StockbasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_cfe961cb-e0cd-4d97-a32a-5cc3d627d46a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_cfe961cb-e0cd-4d97-a32a-5cc3d627d46a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedPaymentArrangementEvergreenProvisionPercentageOfOutstandingShares_3a2d4908-f404-4257-bb9a-d70e8f15414b" xlink:href="mnts-20230930.xsd#mnts_ShareBasedPaymentArrangementEvergreenProvisionPercentageOfOutstandingShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_mnts_ShareBasedPaymentArrangementEvergreenProvisionPercentageOfOutstandingShares_3a2d4908-f404-4257-bb9a-d70e8f15414b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_dc3e6fe6-6075-4779-b77b-b3720ba5755e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_dc3e6fe6-6075-4779-b77b-b3720ba5755e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_91dbd14a-eedb-4932-a034-04ec8dfd00d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_91dbd14a-eedb-4932-a034-04ec8dfd00d7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_ce34a281-f52d-4cd9-9bcd-ab194432a6d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_ce34a281-f52d-4cd9-9bcd-ab194432a6d9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_AccruedEmployeeBenefitsEmployeeStockPurchasePlan_1bb29040-e8bc-4205-a84c-14ad8dfb8f55" xlink:href="mnts-20230930.xsd#mnts_AccruedEmployeeBenefitsEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_mnts_AccruedEmployeeBenefitsEmployeeStockPurchasePlan_1bb29040-e8bc-4205-a84c-14ad8dfb8f55" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_13967d29-79e0-46bf-b398-7ebac7b3a9ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_13967d29-79e0-46bf-b398-7ebac7b3a9ee" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_d9db6871-e6cc-4ea5-bc95-83aaa0fd0d6b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_d9db6871-e6cc-4ea5-bc95-83aaa0fd0d6b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_0128b421-d232-45eb-bdc4-28b6f8d71035" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_0128b421-d232-45eb-bdc4-28b6f8d71035" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_d6bc6ec4-a257-47ef-9b90-cd244e66bc2a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_d6bc6ec4-a257-47ef-9b90-cd244e66bc2a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_c42f39bb-512b-461f-b6ba-d9ff3c8c74a5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_c42f39bb-512b-461f-b6ba-d9ff3c8c74a5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5a2fa20f-1f4f-4765-9a32-4ebee7c008c3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5a2fa20f-1f4f-4765-9a32-4ebee7c008c3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_69bba80d-7f8f-4b31-8a50-dddef04755a0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_69bba80d-7f8f-4b31-8a50-dddef04755a0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_e1a38e9c-2c9b-4105-9ecd-6c0ec51f97f4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_e1a38e9c-2c9b-4105-9ecd-6c0ec51f97f4" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_289d2458-ecd0-4f20-ad48-f8274ac84cfa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_289d2458-ecd0-4f20-ad48-f8274ac84cfa" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_07f6bb4f-36ce-4d4b-9664-32c478e896a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_07f6bb4f-36ce-4d4b-9664-32c478e896a6" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41a4945e-9932-4ad5-8eae-102d282198ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_97c46ef5-1585-494d-a4ee-5e006fbfdee0" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41a4945e-9932-4ad5-8eae-102d282198ee" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_516717ae-cfc7-4475-af3a-9ab90ae1b5bf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41a4945e-9932-4ad5-8eae-102d282198ee" xlink:to="loc_us-gaap_PlanNameAxis_516717ae-cfc7-4475-af3a-9ab90ae1b5bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_516717ae-cfc7-4475-af3a-9ab90ae1b5bf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_516717ae-cfc7-4475-af3a-9ab90ae1b5bf" xlink:to="loc_us-gaap_PlanNameDomain_516717ae-cfc7-4475-af3a-9ab90ae1b5bf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_516717ae-cfc7-4475-af3a-9ab90ae1b5bf" xlink:to="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_A2021EquityIncentivePlanMember_2096c5a4-0ba2-498f-8f4c-bf26ddc54aac" xlink:href="mnts-20230930.xsd#mnts_A2021EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:to="loc_mnts_A2021EquityIncentivePlanMember_2096c5a4-0ba2-498f-8f4c-bf26ddc54aac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_EvergreenPlanMember_dab968d5-6dad-46fb-a05d-74d5e1f9e5bd" xlink:href="mnts-20230930.xsd#mnts_EvergreenPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:to="loc_mnts_EvergreenPlanMember_dab968d5-6dad-46fb-a05d-74d5e1f9e5bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_LegacyStockPlanMember_7745cdad-e7c2-4618-911b-26521171c0d7" xlink:href="mnts-20230930.xsd#mnts_LegacyStockPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:to="loc_mnts_LegacyStockPlanMember_7745cdad-e7c2-4618-911b-26521171c0d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_A2021EmployeeStockPurchasePlanMember_05c59490-8d08-4a3f-acab-682ace3a1b47" xlink:href="mnts-20230930.xsd#mnts_A2021EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:to="loc_mnts_A2021EmployeeStockPurchasePlanMember_05c59490-8d08-4a3f-acab-682ace3a1b47" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_A2022InducementEquityPlanMember_955e9ee5-84b8-42e7-8a7d-229a1f367d2b" xlink:href="mnts-20230930.xsd#mnts_A2022InducementEquityPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_aee15f0f-d342-4c57-8666-680b0af13291" xlink:to="loc_mnts_A2022InducementEquityPlanMember_955e9ee5-84b8-42e7-8a7d-229a1f367d2b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_47232254-adc6-4f00-a20c-4a97b3aea9ac" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41a4945e-9932-4ad5-8eae-102d282198ee" xlink:to="loc_us-gaap_AwardTypeAxis_47232254-adc6-4f00-a20c-4a97b3aea9ac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_47232254-adc6-4f00-a20c-4a97b3aea9ac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_47232254-adc6-4f00-a20c-4a97b3aea9ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_47232254-adc6-4f00-a20c-4a97b3aea9ac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_47232254-adc6-4f00-a20c-4a97b3aea9ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_2479a1ef-f61d-4156-a7e2-eba2d19b4e0d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_2479a1ef-f61d-4156-a7e2-eba2d19b4e0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_1c8aaf39-10fa-4380-ae0d-f9765dbf39ca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:to="loc_us-gaap_EmployeeStockOptionMember_1c8aaf39-10fa-4380-ae0d-f9765dbf39ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_OutstandingVestedAndExpectedToVestRSUsMember_573d97e7-70d6-4fa1-a82a-15c05d0e6a22" xlink:href="mnts-20230930.xsd#mnts_OutstandingVestedAndExpectedToVestRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:to="loc_mnts_OutstandingVestedAndExpectedToVestRSUsMember_573d97e7-70d6-4fa1-a82a-15c05d0e6a22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_3e29908a-b56f-44df-9f8a-be9e9645712a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_652fcca1-45a9-44ef-9bba-88461db21040" xlink:to="loc_us-gaap_PerformanceSharesMember_3e29908a-b56f-44df-9f8a-be9e9645712a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_8eb6de47-f7ed-477e-8b31-44e51fc34a42" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41a4945e-9932-4ad5-8eae-102d282198ee" xlink:to="loc_us-gaap_GranteeStatusAxis_8eb6de47-f7ed-477e-8b31-44e51fc34a42" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_8eb6de47-f7ed-477e-8b31-44e51fc34a42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_8eb6de47-f7ed-477e-8b31-44e51fc34a42" xlink:to="loc_us-gaap_GranteeStatusDomain_8eb6de47-f7ed-477e-8b31-44e51fc34a42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_cff96bc4-e873-4757-bead-b9b260a94e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_8eb6de47-f7ed-477e-8b31-44e51fc34a42" xlink:to="loc_us-gaap_GranteeStatusDomain_cff96bc4-e873-4757-bead-b9b260a94e2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementNonemployeeMember_1e7d884d-a87d-4b69-8b28-cca34d4e82a0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedPaymentArrangementNonemployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_cff96bc4-e873-4757-bead-b9b260a94e2f" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementNonemployeeMember_1e7d884d-a87d-4b69-8b28-cca34d4e82a0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/StockbasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#StockbasedCompensationScheduleofRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/StockbasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7b5192b4-344b-4ff9-84c0-e3145245912e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7b5192b4-344b-4ff9-84c0-e3145245912e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_2dced1a5-cb01-4812-b76d-ba8470c2b0f1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_2dced1a5-cb01-4812-b76d-ba8470c2b0f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_fe768e55-b2f1-43f3-b8b4-92e334c9bcb3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_fe768e55-b2f1-43f3-b8b4-92e334c9bcb3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f2f32f4b-b8f5-4bb0-bb11-33b0eff98cdd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f2f32f4b-b8f5-4bb0-bb11-33b0eff98cdd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_bf197568-5ccf-42a6-9377-49e2608cecac" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2ebf5fda-a58b-4a2a-874c-585da415d432" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_bf197568-5ccf-42a6-9377-49e2608cecac" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_bc080d21-7102-4967-8383-b8d5dabfcd58" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7b5192b4-344b-4ff9-84c0-e3145245912e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_24afacaa-ed5f-4714-96e8-9ee20bc9e22e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_24afacaa-ed5f-4714-96e8-9ee20bc9e22e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_804c3c8d-69f1-41c7-9363-304b51724967" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_804c3c8d-69f1-41c7-9363-304b51724967" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_eccec26c-9048-459b-9608-8fe87a7be1da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_eccec26c-9048-459b-9608-8fe87a7be1da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_22beb1b7-8cb2-495c-b075-92f6b5d432a7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_3ef3366f-4f15-4930-a67f-cf5f126015d1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_22beb1b7-8cb2-495c-b075-92f6b5d432a7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_8649409e-f861-4a57-b0e4-a51e23c5e8d6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_95f4eed7-23ed-490e-bfff-14ceed571fa7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7b5192b4-344b-4ff9-84c0-e3145245912e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_95f4eed7-23ed-490e-bfff-14ceed571fa7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_71d29c6e-5b5e-4a96-9491-b216f42a6231" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_95f4eed7-23ed-490e-bfff-14ceed571fa7" xlink:to="loc_us-gaap_AwardTypeAxis_71d29c6e-5b5e-4a96-9491-b216f42a6231" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71d29c6e-5b5e-4a96-9491-b216f42a6231_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_71d29c6e-5b5e-4a96-9491-b216f42a6231" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71d29c6e-5b5e-4a96-9491-b216f42a6231_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5b401856-b98f-4fd4-a99a-3f64ea5bfc3d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_71d29c6e-5b5e-4a96-9491-b216f42a6231" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5b401856-b98f-4fd4-a99a-3f64ea5bfc3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_84e59039-d3d3-40e5-bc88-4f5d3b7171da" xlink:href="mnts-20230930.xsd#mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5b401856-b98f-4fd4-a99a-3f64ea5bfc3d" xlink:to="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_84e59039-d3d3-40e5-bc88-4f5d3b7171da" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/StockbasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#StockbasedCompensationScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/StockbasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c8283b08-08e1-452d-8903-49be218e3dfa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_3cd4fa1d-c7aa-4e4e-9a0a-55d7eb24c261" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c8283b08-08e1-452d-8903-49be218e3dfa" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_3cd4fa1d-c7aa-4e4e-9a0a-55d7eb24c261" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be84f3b6-3091-49ff-bae8-40772f8b6dc6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c8283b08-08e1-452d-8903-49be218e3dfa" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be84f3b6-3091-49ff-bae8-40772f8b6dc6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_201df176-0390-440e-9494-b84f70b28a8d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be84f3b6-3091-49ff-bae8-40772f8b6dc6" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_201df176-0390-440e-9494-b84f70b28a8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_201df176-0390-440e-9494-b84f70b28a8d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_201df176-0390-440e-9494-b84f70b28a8d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_201df176-0390-440e-9494-b84f70b28a8d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_9c091084-c2cc-49c6-a0c3-11b6218911c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_201df176-0390-440e-9494-b84f70b28a8d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_9c091084-c2cc-49c6-a0c3-11b6218911c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_50e564b6-d32f-4723-8be5-54808adfcb68" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_9c091084-c2cc-49c6-a0c3-11b6218911c0" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_50e564b6-d32f-4723-8be5-54808adfcb68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_fd1dc19f-f4d4-45bc-a4d9-2064ff35c805" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_9c091084-c2cc-49c6-a0c3-11b6218911c0" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_fd1dc19f-f4d4-45bc-a4d9-2064ff35c805" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_c312f3ec-4f17-4b96-8d89-38508859d48e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be84f3b6-3091-49ff-bae8-40772f8b6dc6" xlink:to="loc_us-gaap_AwardTypeAxis_c312f3ec-4f17-4b96-8d89-38508859d48e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c312f3ec-4f17-4b96-8d89-38508859d48e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_c312f3ec-4f17-4b96-8d89-38508859d48e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c312f3ec-4f17-4b96-8d89-38508859d48e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_c312f3ec-4f17-4b96-8d89-38508859d48e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_a31ae3fe-99c0-4bcc-a695-c449b9ab191b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:to="loc_us-gaap_EmployeeStockOptionMember_a31ae3fe-99c0-4bcc-a695-c449b9ab191b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_f7189313-7ee3-472c-b4df-d0d1b6d62491" xlink:href="mnts-20230930.xsd#mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:to="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_f7189313-7ee3-472c-b4df-d0d1b6d62491" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_9c97a51b-ca2f-4bff-a167-1c0292f23e86" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:to="loc_us-gaap_EmployeeStockMember_9c97a51b-ca2f-4bff-a167-1c0292f23e86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_de8a91a7-0fbd-4db1-b9cb-fae2060cf5a3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6c7a16d3-389c-447a-b54d-ae8adf461f0c" xlink:to="loc_us-gaap_PerformanceSharesMember_de8a91a7-0fbd-4db1-b9cb-fae2060cf5a3" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/DilutedEarningsPerShareScheduleofAntidilutiveSharesDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#DilutedEarningsPerShareScheduleofAntidilutiveSharesDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/DilutedEarningsPerShareScheduleofAntidilutiveSharesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_b092d4b7-ba36-4d95-b58a-78a2f6c3b437" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_82c74304-679b-4e2c-a96e-96a86b63c0e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_b092d4b7-ba36-4d95-b58a-78a2f6c3b437" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_82c74304-679b-4e2c-a96e-96a86b63c0e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_fb21f671-3582-457a-8208-9a9c20bc5738" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_b092d4b7-ba36-4d95-b58a-78a2f6c3b437" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_fb21f671-3582-457a-8208-9a9c20bc5738" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_3b3193a9-4d86-45fb-b917-8df3ef43da55" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_fb21f671-3582-457a-8208-9a9c20bc5738" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_3b3193a9-4d86-45fb-b917-8df3ef43da55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_3b3193a9-4d86-45fb-b917-8df3ef43da55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_3b3193a9-4d86-45fb-b917-8df3ef43da55" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_3b3193a9-4d86-45fb-b917-8df3ef43da55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_35d1667c-615d-4d1c-bdb2-b70467f8c452" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_3b3193a9-4d86-45fb-b917-8df3ef43da55" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_35d1667c-615d-4d1c-bdb2-b70467f8c452" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ShareBasedPaymentArrangementOptionsAndUnvestedStockUnitsMember_d809e847-05e1-47a9-b7e1-d95a88bdf004" xlink:href="mnts-20230930.xsd#mnts_ShareBasedPaymentArrangementOptionsAndUnvestedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_35d1667c-615d-4d1c-bdb2-b70467f8c452" xlink:to="loc_mnts_ShareBasedPaymentArrangementOptionsAndUnvestedStockUnitsMember_d809e847-05e1-47a9-b7e1-d95a88bdf004" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_35a8819a-7843-441a-b791-e05dbec82f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_35d1667c-615d-4d1c-bdb2-b70467f8c452" xlink:to="loc_us-gaap_WarrantMember_35a8819a-7843-441a-b791-e05dbec82f8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ContingentSponsorEarnoutSharesMember_e0e6faee-a160-4c44-a898-ce7991c04b7b" xlink:href="mnts-20230930.xsd#mnts_ContingentSponsorEarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_35d1667c-615d-4d1c-bdb2-b70467f8c452" xlink:to="loc_mnts_ContingentSponsorEarnoutSharesMember_e0e6faee-a160-4c44-a898-ce7991c04b7b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/CommitmentandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#CommitmentandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/CommitmentandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationReserve_1541bdaf-fa9a-4e4d-9920-41dc0d7b0f0e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LitigationReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_LitigationReserve_1541bdaf-fa9a-4e4d-9920-41dc0d7b0f0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimatedInsuranceRecoveries_ccf06fc8-772f-4345-9945-1db104b4373c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EstimatedInsuranceRecoveries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_EstimatedInsuranceRecoveries_ccf06fc8-772f-4345-9945-1db104b4373c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EscrowDeposit_b6f38801-7091-4ef1-9671-9ce82a8c82f4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EscrowDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_EscrowDeposit_b6f38801-7091-4ef1-9671-9ce82a8c82f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyLossInPeriod_cbebb3eb-cee0-4157-9551-00f49078f7bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingencyLossInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_LossContingencyLossInPeriod_cbebb3eb-cee0-4157-9551-00f49078f7bb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_1a38c0f6-2729-4cfb-80a7-5fa2084db9bf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_1a38c0f6-2729-4cfb-80a7-5fa2084db9bf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired_aef92407-f7ba-4898-975d-2a5aa6721e31" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_ReverseRecapitalizationPercentageOfVotingInterestsAcquired_aef92407-f7ba-4898-975d-2a5aa6721e31" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationPaymentsToCoFounders_98e2021e-3a3f-46c4-97c3-e8daf43648ea" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationPaymentsToCoFounders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_ReverseRecapitalizationPaymentsToCoFounders_98e2021e-3a3f-46c4-97c3-e8daf43648ea" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5ee77cb9-634e-4f25-9f33-3605248c007b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_5ee77cb9-634e-4f25-9f33-3605248c007b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationContingentConsiderationEquityEarnoutPeriodThresholdBusinessDays_b6d231bf-8a8c-4402-8b52-64515f2a4213" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationContingentConsiderationEquityEarnoutPeriodThresholdBusinessDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_ReverseRecapitalizationContingentConsiderationEquityEarnoutPeriodThresholdBusinessDays_b6d231bf-8a8c-4402-8b52-64515f2a4213" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum_ddd5a5c2-976a-44fc-aa01-e7d52d247436" xlink:href="mnts-20230930.xsd#mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_ProceedsFromSaleOfBusinessAndOtherCapitalRaisingTransactionsMinimum_ddd5a5c2-976a-44fc-aa01-e7d52d247436" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_9b92eae4-412a-4a14-851a-a4788a36bef1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_9b92eae4-412a-4a14-851a-a4788a36bef1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_LossContingencyDamagesSoughtPerBreachValue_0e6c1065-94b1-4bdb-b11b-79775a045598" xlink:href="mnts-20230930.xsd#mnts_LossContingencyDamagesSoughtPerBreachValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_LossContingencyDamagesSoughtPerBreachValue_0e6c1065-94b1-4bdb-b11b-79775a045598" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalFees_b5155fd7-0e25-4512-b9b1-43e7740b68f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LegalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_LegalFees_b5155fd7-0e25-4512-b9b1-43e7740b68f5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_LossContingencyAllegedInvestmentAmount_a6691968-7d17-4f68-b2fc-e007dced4d49" xlink:href="mnts-20230930.xsd#mnts_LossContingencyAllegedInvestmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_mnts_LossContingencyAllegedInvestmentAmount_a6691968-7d17-4f68-b2fc-e007dced4d49" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_ed004eca-9d62-40cb-a846-12f0f3a63f6c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_CommonStockSharesIssued_ed004eca-9d62-40cb-a846-12f0f3a63f6c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesSoughtValue_c9711c1f-72bf-438a-b5ac-97d687c544c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingencyDamagesSoughtValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_LossContingencyDamagesSoughtValue_c9711c1f-72bf-438a-b5ac-97d687c544c0" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_46873775-00a4-48cd-9e50-5a4bc8a2065e" xlink:to="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_0dea8b74-a1b5-4de8-bb72-cba42e3b5480" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:to="loc_srt_LitigationCaseAxis_0dea8b74-a1b5-4de8-bb72-cba42e3b5480" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_0dea8b74-a1b5-4de8-bb72-cba42e3b5480_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_0dea8b74-a1b5-4de8-bb72-cba42e3b5480" xlink:to="loc_srt_LitigationCaseTypeDomain_0dea8b74-a1b5-4de8-bb72-cba42e3b5480_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_0dea8b74-a1b5-4de8-bb72-cba42e3b5480" xlink:to="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_BakerMckenzieMember_e06b9fbf-e8d6-4af4-8e8c-0746a8584eeb" xlink:href="mnts-20230930.xsd#mnts_BakerMckenzieMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:to="loc_mnts_BakerMckenzieMember_e06b9fbf-e8d6-4af4-8e8c-0746a8584eeb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NationalSecurityAgreementMember_035e2539-7961-41e4-bffa-f27125e6ca43" xlink:href="mnts-20230930.xsd#mnts_NationalSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:to="loc_mnts_NationalSecurityAgreementMember_035e2539-7961-41e4-bffa-f27125e6ca43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SAFENoteLitigationMember_8d475c8e-0f57-4ca7-a137-e754ce6e5a3c" xlink:href="mnts-20230930.xsd#mnts_SAFENoteLitigationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:to="loc_mnts_SAFENoteLitigationMember_8d475c8e-0f57-4ca7-a137-e754ce6e5a3c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_StephenJPurcellMember_4354393d-c1f1-474e-8202-686dba31b1db" xlink:href="mnts-20230930.xsd#mnts_StephenJPurcellMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_401fc86a-327e-4397-93f8-ed90374728b4" xlink:to="loc_mnts_StephenJPurcellMember_4354393d-c1f1-474e-8202-686dba31b1db" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_8330d2c9-58dd-44f0-8daa-585ac447ffb1" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_8330d2c9-58dd-44f0-8daa-585ac447ffb1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_8330d2c9-58dd-44f0-8daa-585ac447ffb1_default" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_8330d2c9-58dd-44f0-8daa-585ac447ffb1" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_8330d2c9-58dd-44f0-8daa-585ac447ffb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_9ac963c9-b46c-4301-bfce-7d62dd81a332" xlink:href="mnts-20230930.xsd#mnts_ReverseRecapitalizationConsiderationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeAxis_8330d2c9-58dd-44f0-8daa-585ac447ffb1" xlink:to="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_9ac963c9-b46c-4301-bfce-7d62dd81a332" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ConsiderationDueWithin10BusinessDaysMember_5f5f08e2-9e26-42d0-9eae-b7ab16fbb305" xlink:href="mnts-20230930.xsd#mnts_ConsiderationDueWithin10BusinessDaysMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnts_ReverseRecapitalizationConsiderationTypeDomain_9ac963c9-b46c-4301-bfce-7d62dd81a332" xlink:to="loc_mnts_ConsiderationDueWithin10BusinessDaysMember_5f5f08e2-9e26-42d0-9eae-b7ab16fbb305" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusAxis_584a8019-a4ce-41c5-9257-251e42b26064" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LitigationStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:to="loc_us-gaap_LitigationStatusAxis_584a8019-a4ce-41c5-9257-251e42b26064" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusDomain_584a8019-a4ce-41c5-9257-251e42b26064_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LitigationStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LitigationStatusAxis_584a8019-a4ce-41c5-9257-251e42b26064" xlink:to="loc_us-gaap_LitigationStatusDomain_584a8019-a4ce-41c5-9257-251e42b26064_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusDomain_6d302629-45e6-43bf-a1ce-0627ac968ac3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LitigationStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LitigationStatusAxis_584a8019-a4ce-41c5-9257-251e42b26064" xlink:to="loc_us-gaap_LitigationStatusDomain_6d302629-45e6-43bf-a1ce-0627ac968ac3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PendingLitigationMember_c44464cb-1383-4b79-8747-087cb1e610e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PendingLitigationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationStatusDomain_6d302629-45e6-43bf-a1ce-0627ac968ac3" xlink:to="loc_us-gaap_PendingLitigationMember_c44464cb-1383-4b79-8747-087cb1e610e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_7536428d-0c50-4847-9995-30854d7b3a2e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_63e8f504-06f5-4178-bba9-b4e4ee7a0479" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_7536428d-0c50-4847-9995-30854d7b3a2e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_7536428d-0c50-4847-9995-30854d7b3a2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_7536428d-0c50-4847-9995-30854d7b3a2e" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_7536428d-0c50-4847-9995-30854d7b3a2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_615da24e-86b1-43d6-a6f3-c1d54a4b912f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_7536428d-0c50-4847-9995-30854d7b3a2e" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_615da24e-86b1-43d6-a6f3-c1d54a4b912f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_a3925d27-1007-46c1-a33a-df66c3b3c5e2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_615da24e-86b1-43d6-a6f3-c1d54a4b912f" xlink:to="loc_us-gaap_SubsequentEventMember_a3925d27-1007-46c1-a33a-df66c3b3c5e2" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://momentus.space/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="mnts-20230930.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://momentus.space/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_82858c83-b1c2-4928-973c-8ead9abeba4b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_82858c83-b1c2-4928-973c-8ead9abeba4b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_37c1381a-21f8-4ce1-ace8-21446715dcda" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_37c1381a-21f8-4ce1-ace8-21446715dcda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_77c64003-540c-42da-a8a5-9045b9839d90" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_77c64003-540c-42da-a8a5-9045b9839d90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01b767cf-e25d-4d13-81bd-3a7b0deda884" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01b767cf-e25d-4d13-81bd-3a7b0deda884" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_bdc39e7d-2f93-4cb5-9fd2-7797477023c3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_bdc39e7d-2f93-4cb5-9fd2-7797477023c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_2f0345a2-e193-429f-8697-f7ea983a7731" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_2f0345a2-e193-429f-8697-f7ea983a7731" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_768b6065-4d53-4cb0-8aa3-83088e650841" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_768b6065-4d53-4cb0-8aa3-83088e650841" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromWarrantExercises_1702a702-893b-41f6-874a-b39f5d6f8998" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromWarrantExercises"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_ProceedsFromWarrantExercises_1702a702-893b-41f6-874a-b39f5d6f8998" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForLegalSettlements_2a8f247b-82f8-480f-ad1a-674c460cf009" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsForLegalSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_PaymentsForLegalSettlements_2a8f247b-82f8-480f-ad1a-674c460cf009" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PaymentsForLegalSettlementsByInsuranceCarrier_c78a698c-88ae-4744-a459-6a368ed6fe89" xlink:href="mnts-20230930.xsd#mnts_PaymentsForLegalSettlementsByInsuranceCarrier"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_mnts_PaymentsForLegalSettlementsByInsuranceCarrier_c78a698c-88ae-4744-a459-6a368ed6fe89" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyLossInPeriod_b2a4c4b5-0372-411b-b96f-cc6cfce538a2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingencyLossInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_LossContingencyLossInPeriod_b2a4c4b5-0372-411b-b96f-cc6cfce538a2" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfWarrants_a89837a2-b0b7-4f94-b146-86e9e32bd904" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromIssuanceOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfWarrants_a89837a2-b0b7-4f94-b146-86e9e32bd904" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ClassOfWarrantOrRightConsiderationPerShare_0d84b4c4-5258-4138-aa44-e8f79c438ef7" xlink:href="mnts-20230930.xsd#mnts_ClassOfWarrantOrRightConsiderationPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_mnts_ClassOfWarrantOrRightConsiderationPerShare_0d84b4c4-5258-4138-aa44-e8f79c438ef7" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_7eef9a3e-77b9-455c-babf-fea1af7fa093" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_7eef9a3e-77b9-455c-babf-fea1af7fa093" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_084a8942-2a9d-45ff-a6f5-d47224e797fe" xlink:to="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_64bebfdd-0b9d-426c-8b48-45e3b976418a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_64bebfdd-0b9d-426c-8b48-45e3b976418a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_64bebfdd-0b9d-426c-8b48-45e3b976418a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_64bebfdd-0b9d-426c-8b48-45e3b976418a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_64bebfdd-0b9d-426c-8b48-45e3b976418a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_0ab96105-9a58-4176-8cc6-a62b06b3a809" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_64bebfdd-0b9d-426c-8b48-45e3b976418a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_0ab96105-9a58-4176-8cc6-a62b06b3a809" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_88644ad4-8803-4fab-a82f-5f0e23dda9e7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_0ab96105-9a58-4176-8cc6-a62b06b3a809" xlink:to="loc_us-gaap_SubsequentEventMember_88644ad4-8803-4fab-a82f-5f0e23dda9e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f4d75817-5aff-4665-a38f-ecc7decd3723" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f4d75817-5aff-4665-a38f-ecc7decd3723" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f4d75817-5aff-4665-a38f-ecc7decd3723_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f4d75817-5aff-4665-a38f-ecc7decd3723" xlink:to="loc_us-gaap_ClassOfStockDomain_f4d75817-5aff-4665-a38f-ecc7decd3723_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_3fef3d85-d2af-4321-9338-bad35182f1a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f4d75817-5aff-4665-a38f-ecc7decd3723" xlink:to="loc_us-gaap_ClassOfStockDomain_3fef3d85-d2af-4321-9338-bad35182f1a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_68c81117-655f-4860-a497-18548f32fa0d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_3fef3d85-d2af-4321-9338-bad35182f1a6" xlink:to="loc_us-gaap_CommonClassAMember_68c81117-655f-4860-a497-18548f32fa0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_a947aa6d-a456-4d84-b47e-1407b066fc7c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_a947aa6d-a456-4d84-b47e-1407b066fc7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_a947aa6d-a456-4d84-b47e-1407b066fc7c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a947aa6d-a456-4d84-b47e-1407b066fc7c" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_a947aa6d-a456-4d84-b47e-1407b066fc7c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a947aa6d-a456-4d84-b47e-1407b066fc7c" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_PrefundedWarrantsMember_64e56e6f-59f9-4d86-a7f0-b12dd7749640" xlink:href="mnts-20230930.xsd#mnts_PrefundedWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_PrefundedWarrantsMember_64e56e6f-59f9-4d86-a7f0-b12dd7749640" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_OctoberWarrantsMember_6198daad-0d19-4d69-b95d-cee785202d2e" xlink:href="mnts-20230930.xsd#mnts_OctoberWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_OctoberWarrantsMember_6198daad-0d19-4d69-b95d-cee785202d2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesAWarrantsMember_df686e90-1008-40ea-86e4-8243765a3865" xlink:href="mnts-20230930.xsd#mnts_SeriesAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_SeriesAWarrantsMember_df686e90-1008-40ea-86e4-8243765a3865" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_SeriesBWarrantsMember_60eebac4-4fe1-47ca-9e0d-284dae625592" xlink:href="mnts-20230930.xsd#mnts_SeriesBWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_SeriesBWarrantsMember_60eebac4-4fe1-47ca-9e0d-284dae625592" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_FebruaryClassAWarrantsMember_a261ebbe-a62a-4008-93ea-f2d804489722" xlink:href="mnts-20230930.xsd#mnts_FebruaryClassAWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_FebruaryClassAWarrantsMember_a261ebbe-a62a-4008-93ea-f2d804489722" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_NovemberWarrantsMember_93afc264-3e3c-4d2c-9306-b3d4b22be80d" xlink:href="mnts-20230930.xsd#mnts_NovemberWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_e6d636e8-f222-485b-92bc-9b764e65cac1" xlink:to="loc_mnts_NovemberWarrantsMember_93afc264-3e3c-4d2c-9306-b3d4b22be80d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_3cc32491-0e28-4592-9127-47635ed623c2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_40d4701f-127e-4ff7-82bb-7ca0f62afe00" xlink:to="loc_srt_LitigationCaseAxis_3cc32491-0e28-4592-9127-47635ed623c2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_3cc32491-0e28-4592-9127-47635ed623c2_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_3cc32491-0e28-4592-9127-47635ed623c2" xlink:to="loc_srt_LitigationCaseTypeDomain_3cc32491-0e28-4592-9127-47635ed623c2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_2254e72c-6e4e-4780-8f8a-2c8f649d0aed" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_3cc32491-0e28-4592-9127-47635ed623c2" xlink:to="loc_srt_LitigationCaseTypeDomain_2254e72c-6e4e-4780-8f8a-2c8f649d0aed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_BakerMckenzieMember_f485cba7-5ce6-4501-bc95-8b33c0875253" xlink:href="mnts-20230930.xsd#mnts_BakerMckenzieMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_2254e72c-6e4e-4780-8f8a-2c8f649d0aed" xlink:to="loc_mnts_BakerMckenzieMember_f485cba7-5ce6-4501-bc95-8b33c0875253" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_FounderLitigationMember_d7e812aa-91da-4439-9a8c-3df53da54f20" xlink:href="mnts-20230930.xsd#mnts_FounderLitigationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_2254e72c-6e4e-4780-8f8a-2c8f649d0aed" xlink:to="loc_mnts_FounderLitigationMember_d7e812aa-91da-4439-9a8c-3df53da54f20" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2023/ecd-2023.xsd#AwardTypeOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_EmployeeStockMember" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_ContingentSponsorEarnoutSharesMember_d69f16bb-8d89-4965-bb1d-1745d059100f" xlink:href="mnts-20230930.xsd#mnts_ContingentSponsorEarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_mnts_ContingentSponsorEarnoutSharesMember_d69f16bb-8d89-4965-bb1d-1745d059100f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_d1c70389-c894-44ba-b7ec-54c191c8519f" xlink:href="mnts-20230930.xsd#mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_mnts_RestrictedStockUnitsAndRestrictedStockAwardsMember_d1c70389-c894-44ba-b7ec-54c191c8519f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnts_OutstandingVestedAndExpectedToVestRSUsMember_6aa13a50-e0a9-489e-bef2-5e2739994f88" xlink:href="mnts-20230930.xsd#mnts_OutstandingVestedAndExpectedToVestRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_mnts_OutstandingVestedAndExpectedToVestRSUsMember_6aa13a50-e0a9-489e-bef2-5e2739994f88" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_PerformanceSharesMember" xlink:type="arc" order="9"/>
  </link:definitionLink>
</link:linkbase>
