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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019ecf33-f2dd-756e-8f52-70804bf5f951,g:b287ea41-b3f0-44ce-a173-2cb97160f1c5-->
<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://www.constellium.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINEQUITYunaudited" xlink:type="simple" xlink:href="cstm-20260630.xsd#CONSOLIDATEDSTATEMENTSOFCHANGESINEQUITYunaudited"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINEQUITYunaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ecf33-f47b-7dfc-a135-99ffa44ad30e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ecf33-f47b-75fc-b59b-f36244bf9275" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ecf33-f47b-7dfc-a135-99ffa44ad30e" xlink:to="loc_us-gaap_StatementTable_019ecf33-f47b-75fc-b59b-f36244bf9275" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f47b-7d1e-bc89-93991b84600f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ecf33-f47b-75fc-b59b-f36244bf9275" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f47b-7d1e-bc89-93991b84600f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-7d1e-bc89-93991b84600f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f47b-7d1e-bc89-93991b84600f" xlink:to="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-7d1e-bc89-93991b84600f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f47b-7d1e-bc89-93991b84600f" xlink:to="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019ecf33-f47b-7840-aae1-d0f878974107" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_CommonStockMember_019ecf33-f47b-7840-aae1-d0f878974107" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019ecf33-f47b-7b7e-9650-ef43e0c5ba1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019ecf33-f47b-7b7e-9650-ef43e0c5ba1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f47b-7904-b810-e96cec21f3fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f47b-7904-b810-e96cec21f3fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019ecf33-f47b-7381-9253-7e16944d13e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019ecf33-f47b-7381-9253-7e16944d13e5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019ecf33-f47b-7a71-8d36-b4324d01f5fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_RetainedEarningsMember_019ecf33-f47b-7a71-8d36-b4324d01f5fb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_019ecf33-f47b-7155-85d7-b92be79b12a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f47b-79c1-80ff-d61e639d4df5" xlink:to="loc_us-gaap_NoncontrollingInterestMember_019ecf33-f47b-7155-85d7-b92be79b12a2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ecf33-f47b-7dfc-a135-99ffa44ad30e" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ecf33-f47b-7e04-b7c3-332819bf3fb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ecf33-f47b-7e04-b7c3-332819bf3fb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f47b-7ba2-9339-3cc1e40cc50c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f47b-7ba2-9339-3cc1e40cc50c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019ecf33-f47b-7fb4-bc9d-683f8b9afb67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_ProfitLoss_019ecf33-f47b-7fb4-bc9d-683f8b9afb67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019ecf33-f47b-7d20-89f7-192ac96adcd9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019ecf33-f47b-7d20-89f7-192ac96adcd9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f47b-7335-870d-267846a1e859" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f47b-7335-870d-267846a1e859" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019ecf33-f47b-71a0-b721-663e5ff3b7e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019ecf33-f47b-71a0-b721-663e5ff3b7e8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_019ecf33-f47b-76d2-b47f-d17f754fba6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_019ecf33-f47b-76d2-b47f-d17f754fba6f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019ecf33-f47b-77d4-b94e-bd02c6922297" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019ecf33-f47b-77d4-b94e-bd02c6922297" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_019ecf33-f47b-796e-b5cd-10b0c03fbf67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_019ecf33-f47b-796e-b5cd-10b0c03fbf67" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued_019ecf33-f47b-7d4d-b945-d0c2ecfd25fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued_019ecf33-f47b-7d4d-b945-d0c2ecfd25fe" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityOther_019ecf33-f47b-78e4-b3ea-6f35bcadec23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_StockholdersEquityOther_019ecf33-f47b-78e4-b3ea-6f35bcadec23" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_019ecf33-f47b-70cd-988f-9e4da9d21f95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ecf33-f47b-7da3-b8d7-afdf15992bd9" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_019ecf33-f47b-70cd-988f-9e4da9d21f95" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ecf33-f47b-7f08-ae98-d4dca82e12ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f47b-7f5b-8405-b1137b1071f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/REVENUEDisclosureofrevenuedisaggregatedbyproductDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#REVENUEDisclosureofrevenuedisaggregatedbyproductDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/REVENUEDisclosureofrevenuedisaggregatedbyproductDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019ecf33-f541-7db7-8f64-7d0cf29e3598" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019ecf33-f541-7fb0-b9d6-64bc234d9ac5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ecf33-f541-7db7-8f64-7d0cf29e3598" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019ecf33-f541-7fb0-b9d6-64bc234d9ac5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ecf33-f541-7e79-b074-9f77e81106ac" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019ecf33-f541-7fb0-b9d6-64bc234d9ac5" xlink:to="loc_srt_ProductOrServiceAxis_019ecf33-f541-7e79-b074-9f77e81106ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7e79-b074-9f77e81106ac_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ecf33-f541-7e79-b074-9f77e81106ac" xlink:to="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7e79-b074-9f77e81106ac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ecf33-f541-7e79-b074-9f77e81106ac" xlink:to="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceRolledProductsMember_019ecf33-f541-7226-aa00-dc3f77f9ef62" xlink:href="cstm-20260630.xsd#cstm_AerospaceRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_AerospaceRolledProductsMember_019ecf33-f541-7226-aa00-dc3f77f9ef62" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_TransportationIndustryDefenseAndOtherRolledProductsMember_019ecf33-f541-7617-8ea4-5af95a994545" xlink:href="cstm-20260630.xsd#cstm_TransportationIndustryDefenseAndOtherRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_TransportationIndustryDefenseAndOtherRolledProductsMember_019ecf33-f541-7617-8ea4-5af95a994545" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingRolledProductsMember_019ecf33-f541-71da-808c-6aa4de429491" xlink:href="cstm-20260630.xsd#cstm_PackagingRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_PackagingRolledProductsMember_019ecf33-f541-71da-808c-6aa4de429491" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveRolledProductsMember_019ecf33-f541-7390-adff-2046469c03fb" xlink:href="cstm-20260630.xsd#cstm_AutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_AutomotiveRolledProductsMember_019ecf33-f541-7390-adff-2046469c03fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_SpecialtyAndOtherThinRolledProductsMember_019ecf33-f541-7244-ba80-76067b7ee109" xlink:href="cstm-20260630.xsd#cstm_SpecialtyAndOtherThinRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_SpecialtyAndOtherThinRolledProductsMember_019ecf33-f541-7244-ba80-76067b7ee109" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveExtrudedProductsMember_019ecf33-f541-70da-854c-847c6ef223df" xlink:href="cstm-20260630.xsd#cstm_AutomotiveExtrudedProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_AutomotiveExtrudedProductsMember_019ecf33-f541-70da-854c-847c6ef223df" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_OtherExtrudedProductsMember_019ecf33-f541-7ba9-a28b-d03fde942fb0" xlink:href="cstm-20260630.xsd#cstm_OtherExtrudedProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ecf33-f541-7384-852e-2bce03cadca2" xlink:to="loc_cstm_OtherExtrudedProductsMember_019ecf33-f541-7ba9-a28b-d03fde942fb0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ecf33-f541-7935-b6c9-79c19a92ae1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ecf33-f541-7db7-8f64-7d0cf29e3598" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ecf33-f541-7935-b6c9-79c19a92ae1e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SEGMENTINFORMATIONSegmentRevenueSegmentCostandAdjustedEBITDADetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SEGMENTINFORMATIONSegmentRevenueSegmentCostandAdjustedEBITDADetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SEGMENTINFORMATIONSegmentRevenueSegmentCostandAdjustedEBITDADetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7b8a-8fc3-470b1d4d1abb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7b8a-8fc3-470b1d4d1abb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-760f-95f0-f9bda30ab1a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7b8a-8fc3-470b1d4d1abb" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-760f-95f0-f9bda30ab1a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-760f-95f0-f9bda30ab1a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-760f-95f0-f9bda30ab1a4" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-760f-95f0-f9bda30ab1a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-760f-95f0-f9bda30ab1a4" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-76ce-8a1a-9a41d622fc97" xlink:href="cstm-20260630.xsd#cstm_AerospaceAndTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:to="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-76ce-8a1a-9a41d622fc97" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-749d-8d73-1401ec66053d" xlink:href="cstm-20260630.xsd#cstm_PackagingAndAutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:to="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-749d-8d73-1401ec66053d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7324-8edb-4f57b2b25ed7" xlink:href="cstm-20260630.xsd#cstm_AutomotiveStructuresAndIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:to="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7324-8edb-4f57b2b25ed7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_HoldingCorporateMember_019ecf33-f542-7b82-8f34-b0558fb1f630" xlink:href="cstm-20260630.xsd#cstm_HoldingCorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-770d-82d6-83d6bff06437" xlink:to="loc_cstm_HoldingCorporateMember_019ecf33-f542-7b82-8f34-b0558fb1f630" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7b8a-8fc3-470b1d4d1abb" xlink:to="loc_srt_ConsolidationItemsAxis_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7" xlink:to="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7b0e-8a29-beb776b78221" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019ecf33-f542-7f0a-b8ef-b0ca9e091bd7" xlink:to="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7b0e-8a29-beb776b78221" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019ecf33-f542-7e41-af9d-9ccd4eb6e656" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7b0e-8a29-beb776b78221" xlink:to="loc_us-gaap_OperatingSegmentsMember_019ecf33-f542-7e41-af9d-9ccd4eb6e656" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_019ecf33-f542-7464-ae3d-5bd95de81556" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ecf33-f542-7b0e-8a29-beb776b78221" xlink:to="loc_us-gaap_IntersegmentEliminationMember_019ecf33-f542-7464-ae3d-5bd95de81556" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ecf33-f542-7d04-a4ac-13623a17bbda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ecf33-f542-7d04-a4ac-13623a17bbda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_CostOfMetal_019ecf33-f542-7f3d-b73c-88c9b05dc749" xlink:href="cstm-20260630.xsd#cstm_CostOfMetal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_cstm_CostOfMetal_019ecf33-f542-7f3d-b73c-88c9b05dc749" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductionAndDistributionCosts_019ecf33-f542-7a46-b482-b91695585e61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductionAndDistributionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_us-gaap_ProductionAndDistributionCosts_019ecf33-f542-7a46-b482-b91695585e61" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_OtherExpensesIncludingSellingGeneralAdministrativeAndResearchAndDevelopmentCosts_019ecf33-f542-7b6c-9b6b-f3b25af96d15" xlink:href="cstm-20260630.xsd#cstm_OtherExpensesIncludingSellingGeneralAdministrativeAndResearchAndDevelopmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_cstm_OtherExpensesIncludingSellingGeneralAdministrativeAndResearchAndDevelopmentCosts_019ecf33-f542-7b6c-9b6b-f3b25af96d15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_019ecf33-f542-73c0-a9a7-2b40f3861e2c" xlink:href="cstm-20260630.xsd#cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7811-a65a-edf3a2e3cf8c" xlink:to="loc_cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_019ecf33-f542-73c0-a9a7-2b40f3861e2c" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentAdjustedEBITDAandreconciliationofAdjustedEBITDAtoNetIncomeDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SEGMENTINFORMATIONScheduleofSegmentAdjustedEBITDAandreconciliationofAdjustedEBITDAtoNetIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentAdjustedEBITDAandreconciliationofAdjustedEBITDAtoNetIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-77b6-b480-a76cddabb8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-77b6-b480-a76cddabb8cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7d21-8869-674b7f52d0b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-77b6-b480-a76cddabb8cc" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7d21-8869-674b7f52d0b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7d21-8869-674b7f52d0b4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7d21-8869-674b7f52d0b4" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7d21-8869-674b7f52d0b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7d21-8869-674b7f52d0b4" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-7341-9406-ae471634c94a" xlink:href="cstm-20260630.xsd#cstm_AerospaceAndTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:to="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-7341-9406-ae471634c94a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-780a-a23d-a139e22060e8" xlink:href="cstm-20260630.xsd#cstm_PackagingAndAutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:to="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-780a-a23d-a139e22060e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-77d8-821b-393c4e5db73c" xlink:href="cstm-20260630.xsd#cstm_AutomotiveStructuresAndIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:to="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-77d8-821b-393c4e5db73c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_HoldingCorporateMember_019ecf33-f542-7e87-aea2-8ea22398fbc5" xlink:href="cstm-20260630.xsd#cstm_HoldingCorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7568-8f4b-35984f18bd8c" xlink:to="loc_cstm_HoldingCorporateMember_019ecf33-f542-7e87-aea2-8ea22398fbc5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis_019ecf33-f542-7e06-b972-39cb530a54ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-77b6-b480-a76cddabb8cc" xlink:to="loc_us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis_019ecf33-f542-7e06-b972-39cb530a54ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CatastrophicEventDomain_019ecf33-f542-7e06-b972-39cb530a54ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CatastrophicEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis_019ecf33-f542-7e06-b972-39cb530a54ae" xlink:to="loc_us-gaap_CatastrophicEventDomain_019ecf33-f542-7e06-b972-39cb530a54ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CatastrophicEventDomain_019ecf33-f542-7250-bf6a-9ade4b0b82af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CatastrophicEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis_019ecf33-f542-7e06-b972-39cb530a54ae" xlink:to="loc_us-gaap_CatastrophicEventDomain_019ecf33-f542-7250-bf6a-9ade4b0b82af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_FloodingOfFacilitiesValaisSwitzerlandMember_019ecf33-f542-7b29-a9a3-910e4b284797" xlink:href="cstm-20260630.xsd#cstm_FloodingOfFacilitiesValaisSwitzerlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CatastrophicEventDomain_019ecf33-f542-7250-bf6a-9ade4b0b82af" xlink:to="loc_cstm_FloodingOfFacilitiesValaisSwitzerlandMember_019ecf33-f542-7b29-a9a3-910e4b284797" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_019ecf33-f542-7fb0-90d4-d6462afce1ae" xlink:href="cstm-20260630.xsd#cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_019ecf33-f542-7fb0-90d4-d6462afce1ae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_MetalPriceLagAdjustments_019ecf33-f542-762d-9141-d16fc46e789a" xlink:href="cstm-20260630.xsd#cstm_MetalPriceLagAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_MetalPriceLagAdjustments_019ecf33-f542-762d-9141-d16fc46e789a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019ecf33-f542-70c9-91a7-02b1ad909cd6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019ecf33-f542-70c9-91a7-02b1ad909cd6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentCharges_019ecf33-f542-7780-8369-d1091b7d2b29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_AssetImpairmentCharges_019ecf33-f542-7780-8369-d1091b7d2b29" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ecf33-f542-7090-b4bd-543543a8bbfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ecf33-f542-7090-b4bd-543543a8bbfb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PensionAndOtherPostretirementBenefitCostNonOperatingGains_019ecf33-f542-7180-8e45-33173a0e85aa" xlink:href="cstm-20260630.xsd#cstm_PensionAndOtherPostretirementBenefitCostNonOperatingGains"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_PensionAndOtherPostretirementBenefitCostNonOperatingGains_019ecf33-f542-7180-8e45-33173a0e85aa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019ecf33-f542-7dee-a8ee-a9cfbfd501b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_RestructuringCharges_019ecf33-f542-7dee-a8ee-a9cfbfd501b6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ecf33-f542-710a-adf4-a1381a064e00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnrealizedGainLossOnDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ecf33-f542-710a-adf4-a1381a064e00" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_UnrealizedExchangeGainsLossesFromReMeasurementOfMonetaryAssetsAndLiabilitiesNet_019ecf33-f542-74e5-8da8-e4e4eb6fa4e6" xlink:href="cstm-20260630.xsd#cstm_UnrealizedExchangeGainsLossesFromReMeasurementOfMonetaryAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_UnrealizedExchangeGainsLossesFromReMeasurementOfMonetaryAssetsAndLiabilitiesNet_019ecf33-f542-74e5-8da8-e4e4eb6fa4e6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDispositionOfAssets1_019eff34-8e50-7c7b-8b97-fefeb9849736" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnDispositionOfAssets1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_GainLossOnDispositionOfAssets1_019eff34-8e50-7c7b-8b97-fefeb9849736" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_OtherAdjustments_019ecf33-f542-7a79-93e4-194463313ad3" xlink:href="cstm-20260630.xsd#cstm_OtherAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_OtherAdjustments_019ecf33-f542-7a79-93e4-194463313ad3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ExpensesOnFactoringArrangementsNonOperating_019ecf33-f542-74a8-87df-c6ea396262d6" xlink:href="cstm-20260630.xsd#cstm_ExpensesOnFactoringArrangementsNonOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_ExpensesOnFactoringArrangementsNonOperating_019ecf33-f542-74a8-87df-c6ea396262d6" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_InterestDebtAndOtherExpensesNet_019ecf33-f542-7c41-97fd-7f0b87124452" xlink:href="cstm-20260630.xsd#cstm_InterestDebtAndOtherExpensesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_InterestDebtAndOtherExpensesNet_019ecf33-f542-7c41-97fd-7f0b87124452" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ecf33-f542-747a-b89d-971a14af94c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ecf33-f542-747a-b89d-971a14af94c6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019ecf33-f542-7766-bbc0-b12176db61de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019ecf33-f542-7766-bbc0-b12176db61de" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f542-7ef0-880d-b9c972c11632" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f542-7ef0-880d-b9c972c11632" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_InsuranceProceedsFromCatastrophes_019ecf33-f542-7713-8364-cb6680243dad" xlink:href="cstm-20260630.xsd#cstm_InsuranceProceedsFromCatastrophes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_cstm_InsuranceProceedsFromCatastrophes_019ecf33-f542-7713-8364-cb6680243dad" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossFromCatastrophes_019ecf33-f542-7307-8524-9ee7d6155b27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossFromCatastrophes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7012-aa73-69ca0cd966d3" xlink:to="loc_us-gaap_LossFromCatastrophes_019ecf33-f542-7307-8524-9ee7d6155b27" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentCapitalExpendituresDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SEGMENTINFORMATIONScheduleofSegmentCapitalExpendituresDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentCapitalExpendituresDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7f17-ab99-58dc637c62d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-70f9-8137-2bd42ced4c15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7f17-ab99-58dc637c62d5" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-70f9-8137-2bd42ced4c15" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-77df-bf18-dab583c1d873" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-70f9-8137-2bd42ced4c15" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-77df-bf18-dab583c1d873" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-77df-bf18-dab583c1d873_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-77df-bf18-dab583c1d873" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-77df-bf18-dab583c1d873_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-77df-bf18-dab583c1d873" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-794d-b262-5f9d9ef6850e" xlink:href="cstm-20260630.xsd#cstm_AerospaceAndTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:to="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-794d-b262-5f9d9ef6850e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-7583-a091-d03adb8aa72f" xlink:href="cstm-20260630.xsd#cstm_PackagingAndAutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:to="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-7583-a091-d03adb8aa72f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7646-b990-5775a7ebbda5" xlink:href="cstm-20260630.xsd#cstm_AutomotiveStructuresAndIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:to="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7646-b990-5775a7ebbda5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_HoldingCorporateMember_019ecf76-a75f-7fb5-822c-59da42dd1e8b" xlink:href="cstm-20260630.xsd#cstm_HoldingCorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7dfa-a379-2d5f6bf47a64" xlink:to="loc_cstm_HoldingCorporateMember_019ecf76-a75f-7fb5-822c-59da42dd1e8b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019ecf33-f542-7774-b081-15bb23682f52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7f17-ab99-58dc637c62d5" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019ecf33-f542-7774-b081-15bb23682f52" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentdepreciationamortizationandimpairmentexpenseDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SEGMENTINFORMATIONScheduleofSegmentdepreciationamortizationandimpairmentexpenseDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentdepreciationamortizationandimpairmentexpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-75bb-9f85-3af71a5aef95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7bdb-b579-9a114d8bdabb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-75bb-9f85-3af71a5aef95" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7bdb-b579-9a114d8bdabb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7b25-af7c-18170d5600bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-7bdb-b579-9a114d8bdabb" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7b25-af7c-18170d5600bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7b25-af7c-18170d5600bc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7b25-af7c-18170d5600bc" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7b25-af7c-18170d5600bc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7b25-af7c-18170d5600bc" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-7c89-a9c1-c18844c8f523" xlink:href="cstm-20260630.xsd#cstm_AerospaceAndTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:to="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-7c89-a9c1-c18844c8f523" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-7df6-9fbb-01aa6f773dab" xlink:href="cstm-20260630.xsd#cstm_PackagingAndAutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:to="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-7df6-9fbb-01aa6f773dab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-71ed-aabf-a43545aa729a" xlink:href="cstm-20260630.xsd#cstm_AutomotiveStructuresAndIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:to="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-71ed-aabf-a43545aa729a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_HoldingCorporateMember_019ecf33-f542-7f88-a979-20fd8fd5b04e" xlink:href="cstm-20260630.xsd#cstm_HoldingCorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7c92-9f0a-bedff10cabbd" xlink:to="loc_cstm_HoldingCorporateMember_019ecf33-f542-7f88-a979-20fd8fd5b04e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_DepreciationAmortizationAndImpairmentCharge_019ecf33-f542-76eb-b3ad-417741d232e7" xlink:href="cstm-20260630.xsd#cstm_DepreciationAmortizationAndImpairmentCharge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-75bb-9f85-3af71a5aef95" xlink:to="loc_cstm_DepreciationAmortizationAndImpairmentCharge_019ecf33-f542-76eb-b3ad-417741d232e7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentAssetsDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SEGMENTINFORMATIONScheduleofSegmentAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SEGMENTINFORMATIONScheduleofSegmentAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-74ee-b440-bf84c870be7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-74ee-b440-bf84c870be7e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7dc9-a881-ceaac36b7900" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-74ee-b440-bf84c870be7e" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7dc9-a881-ceaac36b7900" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7dc9-a881-ceaac36b7900_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7dc9-a881-ceaac36b7900" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7dc9-a881-ceaac36b7900_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ecf33-f542-7dc9-a881-ceaac36b7900" xlink:to="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-72ba-bf87-1d86b4b874f9" xlink:href="cstm-20260630.xsd#cstm_AerospaceAndTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:to="loc_cstm_AerospaceAndTransportationMember_019ecf33-f542-72ba-bf87-1d86b4b874f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-78c0-bb0b-1e146b99c575" xlink:href="cstm-20260630.xsd#cstm_PackagingAndAutomotiveRolledProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:to="loc_cstm_PackagingAndAutomotiveRolledProductsMember_019ecf33-f542-78c0-bb0b-1e146b99c575" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7820-a20f-5f06b1dc47b4" xlink:href="cstm-20260630.xsd#cstm_AutomotiveStructuresAndIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:to="loc_cstm_AutomotiveStructuresAndIndustryMember_019ecf33-f542-7820-a20f-5f06b1dc47b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_HoldingCorporateMember_019ecf33-f542-789b-9cbd-c5bc2e61858f" xlink:href="cstm-20260630.xsd#cstm_HoldingCorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ecf33-f542-7017-8690-428acfadf858" xlink:to="loc_cstm_HoldingCorporateMember_019ecf33-f542-789b-9cbd-c5bc2e61858f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019ecf33-f542-70b6-b853-30bf13f06965" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ecf33-f542-74ee-b440-bf84c870be7e" xlink:to="loc_srt_ConsolidationItemsAxis_019ecf33-f542-70b6-b853-30bf13f06965" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ecf33-f542-70b6-b853-30bf13f06965_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019ecf33-f542-70b6-b853-30bf13f06965" xlink:to="loc_srt_ConsolidationItemsDomain_019ecf33-f542-70b6-b853-30bf13f06965_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ecf33-f542-707f-8212-9e931f7ff9cc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019ecf33-f542-70b6-b853-30bf13f06965" xlink:to="loc_srt_ConsolidationItemsDomain_019ecf33-f542-707f-8212-9e931f7ff9cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019ecf33-f542-7ecd-aed7-3cb8a3f075e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ecf33-f542-707f-8212-9e931f7ff9cc" xlink:to="loc_us-gaap_OperatingSegmentsMember_019ecf33-f542-7ecd-aed7-3cb8a3f075e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019ecf33-f542-7d8b-88b6-460638eed0d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ecf33-f542-707f-8212-9e931f7ff9cc" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019ecf33-f542-7d8b-88b6-460638eed0d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019ecf33-f542-7092-a885-d552adc8292b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:to="loc_us-gaap_Assets_019ecf33-f542-7092-a885-d552adc8292b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_019ecf33-f542-7f5c-aeb5-38c268ad5682" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_019ecf33-f542-7f5c-aeb5-38c268ad5682" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ecf33-f542-7d49-8ef1-480743eda6d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ecf33-f542-7d49-8ef1-480743eda6d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_DerivativesAndOtherFinancialAssets_019ecf33-f542-7122-8f60-53189cd819e5" xlink:href="cstm-20260630.xsd#cstm_DerivativesAndOtherFinancialAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ecf33-f542-7853-adf5-084fe4f2a397" xlink:to="loc_cstm_DerivativesAndOtherFinancialAssets_019ecf33-f542-7122-8f60-53189cd819e5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCECOSTSNETDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCECOSTSNETDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCECOSTSNETDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-712d-8689-3c8973182e33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-712d-8689-3c8973182e33" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-758e-8f5b-d0ef4d699a8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-712d-8689-3c8973182e33" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-758e-8f5b-d0ef4d699a8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-758e-8f5b-d0ef4d699a8a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-758e-8f5b-d0ef4d699a8a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-758e-8f5b-d0ef4d699a8a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7c6e-864e-7b5e76af3fee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-758e-8f5b-d0ef4d699a8a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7c6e-864e-7b5e76af3fee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019ecf33-f542-7c64-92ac-9a292c322b4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7c6e-864e-7b5e76af3fee" xlink:to="loc_us-gaap_SeniorNotesMember_019ecf33-f542-7c64-92ac-9a292c322b4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019ecf33-f542-7966-89d8-06897c957394" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_InterestExpenseDebt_019ecf33-f542-7966-89d8-06897c957394" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseInterestExpense_019ecf7a-6e0f-75d6-acb6-68e21bd66b56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_FinanceLeaseInterestExpense_019ecf7a-6e0f-75d6-acb6-68e21bd66b56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_DefinedBenefitPlanInterestCostNetOfExpectedReturnOnPlanAssets_019ecf33-f542-7f71-bc42-212942b6c508" xlink:href="cstm-20260630.xsd#cstm_DefinedBenefitPlanInterestCostNetOfExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_cstm_DefinedBenefitPlanInterestCostNetOfExpectedReturnOnPlanAssets_019ecf33-f542-7f71-bc42-212942b6c508" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDerivativeInstrumentsNetPretax_019ecf33-f542-72e8-ac86-ddb0cc81ea1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnDerivativeInstrumentsNetPretax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_GainLossOnDerivativeInstrumentsNetPretax_019ecf33-f542-72e8-ac86-ddb0cc81ea1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_RealizedAndUnrealizedExchangeGainsLossesOnFinancingActivitiesNet_019ecf33-f542-7aab-a9c7-61dc2a019398" xlink:href="cstm-20260630.xsd#cstm_RealizedAndUnrealizedExchangeGainsLossesOnFinancingActivitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_cstm_RealizedAndUnrealizedExchangeGainsLossesOnFinancingActivitiesNet_019ecf33-f542-7aab-a9c7-61dc2a019398" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseOther_019ecf33-f542-75fd-8872-cbb06fe7783e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_InterestExpenseOther_019ecf33-f542-75fd-8872-cbb06fe7783e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_CapitalizedBorrowingCosts_019ecf33-f542-7697-9baa-66280efaebfb" xlink:href="cstm-20260630.xsd#cstm_CapitalizedBorrowingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_cstm_CapitalizedBorrowingCosts_019ecf33-f542-7697-9baa-66280efaebfb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndDebtExpense_019ecf33-f542-77ac-bf68-7398ec86278f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndDebtExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_InterestAndDebtExpense_019ecf33-f542-77ac-bf68-7398ec86278f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_InterestDebtAndOtherExpensesNet_019ecf33-f542-7897-87b3-77e51f6a85ab" xlink:href="cstm-20260630.xsd#cstm_InterestDebtAndOtherExpensesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_cstm_InterestDebtAndOtherExpensesNet_019ecf33-f542-7897-87b3-77e51f6a85ab" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseBorrowings_019ecf33-f542-7c8f-8df9-7f4f1a639c3b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_us-gaap_InterestExpenseBorrowings_019ecf33-f542-7c8f-8df9-7f4f1a639c3b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PropertyPlantAndEquipmentInterestCapitalizationPercentage_019ecf33-f542-7eea-bbc0-dd7b4bb014a6" xlink:href="cstm-20260630.xsd#cstm_PropertyPlantAndEquipmentInterestCapitalizationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7e11-881c-ee30ea6de3cc" xlink:to="loc_cstm_PropertyPlantAndEquipmentInterestCapitalizationPercentage_019ecf33-f542-7eea-bbc0-dd7b4bb014a6" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/DEBTScheduleofdebtinstrumentsDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#DEBTScheduleofdebtinstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/DEBTScheduleofdebtinstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-75d5-8dee-445428d59e50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-75d5-8dee-445428d59e50" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7c50-b8f8-4189945c7eeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-75d5-8dee-445428d59e50" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7c50-b8f8-4189945c7eeb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7c50-b8f8-4189945c7eeb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7c50-b8f8-4189945c7eeb" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7c50-b8f8-4189945c7eeb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7c50-b8f8-4189945c7eeb" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PanU.S.ABLFacilityMember_019ecf33-f542-7d08-a004-0a45a972ec5f" xlink:href="cstm-20260630.xsd#cstm_PanU.S.ABLFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_PanU.S.ABLFacilityMember_019ecf33-f542-7d08-a004-0a45a972ec5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019ecf33-f542-7657-9e59-63cd16991ee0" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019ecf33-f542-7657-9e59-63cd16991ee0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember_019ecf33-f542-79f4-817c-bd3b5f0ac9ed" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember_019ecf33-f542-79f4-817c-bd3b5f0ac9ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember_019ecf33-f542-7b14-a3c5-b37d43f11104" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember_019ecf33-f542-7b14-a3c5-b37d43f11104" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember_019ecf33-f542-75b4-8692-6961e456bad2" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember_019ecf33-f542-75b4-8692-6961e456bad2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032300MillionMember_019ecf33-f542-79f1-b26d-7e4cd9d64c98" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032300MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032300MillionMember_019ecf33-f542-79f1-b26d-7e4cd9d64c98" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ShortTermAndLongTermLoansMember_019ecf33-f542-7380-863c-144c3222dfb6" xlink:href="cstm-20260630.xsd#cstm_ShortTermAndLongTermLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7f47-9433-1f9839b1fd88" xlink:to="loc_cstm_ShortTermAndLongTermLoansMember_019ecf33-f542-7380-863c-144c3222dfb6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-77fe-9aed-b31f1191347c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-75d5-8dee-445428d59e50" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-77fe-9aed-b31f1191347c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-77fe-9aed-b31f1191347c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-77fe-9aed-b31f1191347c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-77fe-9aed-b31f1191347c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7a68-9573-fed0bac68770" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-77fe-9aed-b31f1191347c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7a68-9573-fed0bac68770" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ecf33-f542-72e9-90bf-b885194858b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7a68-9573-fed0bac68770" xlink:to="loc_us-gaap_LineOfCreditMember_019ecf33-f542-72e9-90bf-b885194858b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019ecf33-f542-72c2-aa51-690f804492b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7a68-9573-fed0bac68770" xlink:to="loc_us-gaap_SeniorNotesMember_019ecf33-f542-72c2-aa51-690f804492b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019ecf33-f542-7e04-8e10-abfa78da61ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019ecf33-f542-7e04-8e10-abfa78da61ad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ecf33-f542-74f0-8836-f39fbaa121f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ecf33-f542-74f0-8836-f39fbaa121f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019ecf33-f542-70e0-8d5b-100b97e3eb2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019ecf33-f542-70e0-8d5b-100b97e3eb2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019ecf33-f542-792a-bf9e-b93062ef040d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019ecf33-f542-792a-bf9e-b93062ef040d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_BorrowingsInterestAccrued_019ecf33-f542-7824-aae6-d99f70e91c83" xlink:href="cstm-20260630.xsd#cstm_BorrowingsInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_cstm_BorrowingsInterestAccrued_019ecf33-f542-7824-aae6-d99f70e91c83" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019ecf33-f542-7e62-9c3f-204bd013525a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_LongTermDebt_019ecf33-f542-7e62-9c3f-204bd013525a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_FinanceLeaseFaceValue_019f8f1c-82b8-7b23-bf37-c6d23281357c" xlink:href="cstm-20260630.xsd#cstm_FinanceLeaseFaceValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_cstm_FinanceLeaseFaceValue_019f8f1c-82b8-7b23-bf37-c6d23281357c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019ecf33-f542-7fa4-81b3-649332987b47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_FinanceLeaseLiability_019ecf33-f542-7fa4-81b3-649332987b47" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_FinanceLeaseArrangementFees_019ecf33-f542-7817-8a7a-cf31703b3014" xlink:href="cstm-20260630.xsd#cstm_FinanceLeaseArrangementFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_cstm_FinanceLeaseArrangementFees_019ecf33-f542-7817-8a7a-cf31703b3014" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_InterestAccruedOnLeaseLiabilities_019ecf33-f542-74dd-9bf4-c7b786046fa3" xlink:href="cstm-20260630.xsd#cstm_InterestAccruedOnLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_cstm_InterestAccruedOnLeaseLiabilities_019ecf33-f542-74dd-9bf4-c7b786046fa3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019ecf33-f542-7541-a00a-aa07e9ff075c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019ecf33-f542-7541-a00a-aa07e9ff075c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ecf33-f542-7572-84d3-5130b4655df0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-779c-b2ee-9c89529df13c" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ecf33-f542-7572-84d3-5130b4655df0" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ecf33-f542-7329-a6e1-a0ee28604622" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ecf33-f542-7572-84d3-5130b4655df0" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ecf33-f542-7329-a6e1-a0ee28604622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtCurrent_019ecf33-f542-72a1-b7a9-c23b96d7422e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ecf33-f542-7572-84d3-5130b4655df0" xlink:to="loc_us-gaap_DebtCurrent_019ecf33-f542-72a1-b7a9-c23b96d7422e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/DEBTNarrativeDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#DEBTNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/DEBTNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7feb-9063-6d6c2aceb27d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7feb-9063-6d6c2aceb27d" xlink:to="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7242-bedb-965fc940c9d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7242-bedb-965fc940c9d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7242-bedb-965fc940c9d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7242-bedb-965fc940c9d1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-7242-bedb-965fc940c9d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ecf33-f542-7242-bedb-965fc940c9d1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019ecf33-f542-7cc0-b165-0f01160c9953" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019ecf33-f542-7cc0-b165-0f01160c9953" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember_019ecf33-f542-75e5-bf12-a243a21ff0ff" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedFebruary2021Due2029500MillionMember_019ecf33-f542-75e5-bf12-a243a21ff0ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember_019ecf33-f542-7996-b89b-781b8476c1b5" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2021Due2029300MillionMember_019ecf33-f542-7996-b89b-781b8476c1b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember_019ecf33-f542-73d9-8f48-495c523fafb9" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedAugust2024Due2032350MillionMember_019ecf33-f542-73d9-8f48-495c523fafb9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_FrenchInventoryFacilityMember_019ecf33-f542-7b4f-90f1-d42de153a832" xlink:href="cstm-20260630.xsd#cstm_FrenchInventoryFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ecf33-f542-74e0-8449-ced723ff7afd" xlink:to="loc_cstm_FrenchInventoryFacilityMember_019ecf33-f542-7b4f-90f1-d42de153a832" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-70e3-93b6-71aa1a2e4681" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-70e3-93b6-71aa1a2e4681" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-70e3-93b6-71aa1a2e4681_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-70e3-93b6-71aa1a2e4681" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-70e3-93b6-71aa1a2e4681_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-727a-ab01-f66470b772d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-70e3-93b6-71aa1a2e4681" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-727a-ab01-f66470b772d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019ecf33-f542-7346-af50-6f13833f254c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-727a-ab01-f66470b772d3" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019ecf33-f542-7346-af50-6f13833f254c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-78f9-a970-5c551334ee2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-78f9-a970-5c551334ee2f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-78f9-a970-5c551334ee2f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-78f9-a970-5c551334ee2f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-78f9-a970-5c551334ee2f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7763-b304-3e8f0cb45c5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ecf33-f542-78f9-a970-5c551334ee2f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7763-b304-3e8f0cb45c5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019ecf33-f542-7238-be7e-940576f64715" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ecf33-f542-7763-b304-3e8f0cb45c5c" xlink:to="loc_us-gaap_SeniorNotesMember_019ecf33-f542-7238-be7e-940576f64715" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ecf33-f542-7b79-8837-3b818d037556" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ecf33-f542-7893-9be1-123fa846e663" xlink:to="loc_us-gaap_CreditFacilityAxis_019ecf33-f542-7b79-8837-3b818d037556" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ecf33-f542-7b79-8837-3b818d037556_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ecf33-f542-7b79-8837-3b818d037556" xlink:to="loc_us-gaap_CreditFacilityDomain_019ecf33-f542-7b79-8837-3b818d037556_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ecf33-f542-74b3-81a4-5fbb4cbe97ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ecf33-f542-7b79-8837-3b818d037556" xlink:to="loc_us-gaap_CreditFacilityDomain_019ecf33-f542-74b3-81a4-5fbb4cbe97ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019ecf33-f542-7dbb-8b34-b4e13a82ea71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ecf33-f542-74b3-81a4-5fbb4cbe97ec" xlink:to="loc_us-gaap_SecuredDebtMember_019ecf33-f542-7dbb-8b34-b4e13a82ea71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_LongTermDebtFairValueInPercentageOfFaceValue_019ecf33-f542-7813-b5e5-8111601b533d" xlink:href="cstm-20260630.xsd#cstm_LongTermDebtFairValueInPercentageOfFaceValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7feb-9063-6d6c2aceb27d" xlink:to="loc_cstm_LongTermDebtFairValueInPercentageOfFaceValue_019ecf33-f542-7813-b5e5-8111601b533d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019ecf33-f542-797c-8204-0c31bab06b1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7feb-9063-6d6c2aceb27d" xlink:to="loc_us-gaap_LongTermDebtFairValue_019ecf33-f542-797c-8204-0c31bab06b1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019ecf33-f542-7133-a3c3-949a286e0d88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ecf33-f542-7feb-9063-6d6c2aceb27d" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019ecf33-f542-7133-a3c3-949a286e0d88" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofderivativesinstrumentspresentedatfairvalueintheConsolidatedBalanceSheetDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCIALINSTRUMENTSScheduleofderivativesinstrumentspresentedatfairvalueintheConsolidatedBalanceSheetDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofderivativesinstrumentspresentedatfairvalueintheConsolidatedBalanceSheetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7d7c-a8a3-e2076452e300" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7d7c-a8a3-e2076452e300" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7e0c-98b8-1488e4a84c44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7d7c-a8a3-e2076452e300" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7e0c-98b8-1488e4a84c44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7e0c-98b8-1488e4a84c44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7e0c-98b8-1488e4a84c44" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7e0c-98b8-1488e4a84c44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7ba2-a10e-9f4c353bf08c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7e0c-98b8-1488e4a84c44" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7ba2-a10e-9f4c353bf08c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7ac4-bfbe-87045f7973c0" xlink:href="cstm-20260630.xsd#cstm_ForeignExchangeContractCommercialDerivativesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7ba2-a10e-9f4c353bf08c" xlink:to="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7ac4-bfbe-87045f7973c0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnergyRelatedDerivativeMember_019ecf33-f542-7e66-9672-9902f19b006f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnergyRelatedDerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7ba2-a10e-9f4c353bf08c" xlink:to="loc_us-gaap_EnergyRelatedDerivativeMember_019ecf33-f542-7e66-9672-9902f19b006f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_CommodityDerivativesMetalNettingMember_019ecf33-f542-770f-b211-cd7f5dcd8c78" xlink:href="cstm-20260630.xsd#cstm_CommodityDerivativesMetalNettingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7ba2-a10e-9f4c353bf08c" xlink:to="loc_cstm_CommodityDerivativesMetalNettingMember_019ecf33-f542-770f-b211-cd7f5dcd8c78" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7fdc-95ad-a96ad6b29bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7d7c-a8a3-e2076452e300" xlink:to="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7fdc-95ad-a96ad6b29bd2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7fdc-95ad-a96ad6b29bd2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7fdc-95ad-a96ad6b29bd2" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7fdc-95ad-a96ad6b29bd2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-782b-83b6-ed6fcedf125f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7fdc-95ad-a96ad6b29bd2" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-782b-83b6-ed6fcedf125f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-7e39-bbd9-6e0dddcaaf58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-782b-83b6-ed6fcedf125f" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-7e39-bbd9-6e0dddcaaf58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019ecf33-f542-77a0-a1dd-4a1250b840fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-782b-83b6-ed6fcedf125f" xlink:to="loc_us-gaap_NondesignatedMember_019ecf33-f542-77a0-a1dd-4a1250b840fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsNoncurrent_019ecf33-f542-7aee-aae6-91c5bbbebd80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeAssetsNoncurrent_019ecf33-f542-7aee-aae6-91c5bbbebd80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsCurrent_019ecf33-f542-7d90-aa76-c1b44166093b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeAssetsCurrent_019ecf33-f542-7d90-aa76-c1b44166093b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019ecf33-f542-7e5f-86d4-d3cf83e82088" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeAssets_019ecf33-f542-7e5f-86d4-d3cf83e82088" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_019ecf33-f542-7ef2-bab8-fd1a84d258de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_019ecf33-f542-7ef2-bab8-fd1a84d258de" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesCurrent_019ecf33-f542-7b90-b6e4-7a19a6f5108b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeLiabilitiesCurrent_019ecf33-f542-7b90-b6e4-7a19a6f5108b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019ecf33-f542-77c8-9712-6e9ba1edac69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-729b-b20b-1d8bf8024411" xlink:to="loc_us-gaap_DerivativeLiabilities_019ecf33-f542-77c8-9712-6e9ba1edac69" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleoffairvaluehierarchyDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCIALINSTRUMENTSScheduleoffairvaluehierarchyDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleoffairvaluehierarchyDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-730c-9154-90554ca05cf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7fb1-b241-1d4782ddb3f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-730c-9154-90554ca05cf7" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7fb1-b241-1d4782ddb3f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-73cd-8f69-57a1af34bd7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7fb1-b241-1d4782ddb3f9" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-73cd-8f69-57a1af34bd7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-73cd-8f69-57a1af34bd7b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-73cd-8f69-57a1af34bd7b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-73cd-8f69-57a1af34bd7b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-71db-a64d-6fc0ea6f52c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ecf33-f542-73cd-8f69-57a1af34bd7b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-71db-a64d-6fc0ea6f52c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019ecf33-f542-7791-87e7-9e793b346c8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-71db-a64d-6fc0ea6f52c7" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019ecf33-f542-7791-87e7-9e793b346c8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019ecf33-f542-7173-ac6a-75b65834e13f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-71db-a64d-6fc0ea6f52c7" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019ecf33-f542-7173-ac6a-75b65834e13f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019ecf33-f542-7532-8b58-ea526fbbaaa8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ecf33-f542-71db-a64d-6fc0ea6f52c7" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019ecf33-f542-7532-8b58-ea526fbbaaa8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019ecf33-f542-7c8b-be3f-9718d23dc3dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-730c-9154-90554ca05cf7" xlink:to="loc_us-gaap_DerivativeAssets_019ecf33-f542-7c8b-be3f-9718d23dc3dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019ecf33-f542-744c-bbee-a1230475bde1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-730c-9154-90554ca05cf7" xlink:to="loc_us-gaap_DerivativeLiabilities_019ecf33-f542-744c-bbee-a1230475bde1" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofnotionalamountsofoutstandingderivativesDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCIALINSTRUMENTSScheduleofnotionalamountsofoutstandingderivativesDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofnotionalamountsofoutstandingderivativesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-76cf-ab8e-5321230611e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-76cf-ab8e-5321230611e4" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_MaturityAxis_019ecf33-f542-7105-ba4b-2661703b231e" xlink:href="cstm-20260630.xsd#cstm_MaturityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:to="loc_cstm_MaturityAxis_019ecf33-f542-7105-ba4b-2661703b231e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_MaturityDomain_019ecf33-f542-7105-ba4b-2661703b231e_default" xlink:href="cstm-20260630.xsd#cstm_MaturityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_cstm_MaturityAxis_019ecf33-f542-7105-ba4b-2661703b231e" xlink:to="loc_cstm_MaturityDomain_019ecf33-f542-7105-ba4b-2661703b231e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_MaturityDomain_019ecf33-f542-75c8-b494-1340ebd6d567" xlink:href="cstm-20260630.xsd#cstm_MaturityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_cstm_MaturityAxis_019ecf33-f542-7105-ba4b-2661703b231e" xlink:to="loc_cstm_MaturityDomain_019ecf33-f542-75c8-b494-1340ebd6d567" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_NotLaterThanOneYearMember_019ecf33-f542-7439-aaa9-4bc99f412a57" xlink:href="cstm-20260630.xsd#cstm_NotLaterThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cstm_MaturityDomain_019ecf33-f542-75c8-b494-1340ebd6d567" xlink:to="loc_cstm_NotLaterThanOneYearMember_019ecf33-f542-7439-aaa9-4bc99f412a57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_LaterThanOneYearMember_019ecf33-f542-7662-ba30-4037a2a2e541" xlink:href="cstm-20260630.xsd#cstm_LaterThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cstm_MaturityDomain_019ecf33-f542-75c8-b494-1340ebd6d567" xlink:to="loc_cstm_LaterThanOneYearMember_019ecf33-f542-7662-ba30-4037a2a2e541" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7596-a4b1-b984ea62f158" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7596-a4b1-b984ea62f158" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7596-a4b1-b984ea62f158_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7596-a4b1-b984ea62f158" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7596-a4b1-b984ea62f158_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7596-a4b1-b984ea62f158" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_EURUSDCurrencyForwardDerivativeMember_019ecf33-f542-7fed-94d3-3b98bc298b81" xlink:href="cstm-20260630.xsd#cstm_EURUSDCurrencyForwardDerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_EURUSDCurrencyForwardDerivativeMember_019ecf33-f542-7fed-94d3-3b98bc298b81" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_EURCHFCurrencyForwardDerivativeMember_019ecf33-f542-7660-ba82-6437e9e5560d" xlink:href="cstm-20260630.xsd#cstm_EURCHFCurrencyForwardDerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_EURCHFCurrencyForwardDerivativeMember_019ecf33-f542-7660-ba82-6437e9e5560d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_EURCZKCurrencyForwardDerivativeMember_019ecf33-f542-7ff0-a6cf-ebb6f2acf130" xlink:href="cstm-20260630.xsd#cstm_EURCZKCurrencyForwardDerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_EURCZKCurrencyForwardDerivativeMember_019ecf33-f542-7ff0-a6cf-ebb6f2acf130" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_EUROtherCurrenciesCurrencyForwardDerivativeMember_019ecf33-f542-7615-b692-7200afa6b7c0" xlink:href="cstm-20260630.xsd#cstm_EUROtherCurrenciesCurrencyForwardDerivativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_EUROtherCurrenciesCurrencyForwardDerivativeMember_019ecf33-f542-7615-b692-7200afa6b7c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_CommodityDerivativesMetalNettingMember_019ecf33-f542-751f-b666-682428810b31" xlink:href="cstm-20260630.xsd#cstm_CommodityDerivativesMetalNettingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_CommodityDerivativesMetalNettingMember_019ecf33-f542-751f-b666-682428810b31" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_NaturalGasDerivativeNettingMember_019ecf33-f542-7958-b771-64e2fb794c68" xlink:href="cstm-20260630.xsd#cstm_NaturalGasDerivativeNettingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7d53-8953-3fad0de93167" xlink:to="loc_cstm_NaturalGasDerivativeNettingMember_019ecf33-f542-7958-b771-64e2fb794c68" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PositionAxis_019ecf33-f542-7207-b499-9488dadbecaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PositionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:to="loc_us-gaap_PositionAxis_019ecf33-f542-7207-b499-9488dadbecaa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PositionDomain_019ecf33-f542-7207-b499-9488dadbecaa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PositionAxis_019ecf33-f542-7207-b499-9488dadbecaa" xlink:to="loc_us-gaap_PositionDomain_019ecf33-f542-7207-b499-9488dadbecaa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PositionDomain_019ecf33-f542-71bc-a687-fb1b89e71856" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PositionAxis_019ecf33-f542-7207-b499-9488dadbecaa" xlink:to="loc_us-gaap_PositionDomain_019ecf33-f542-71bc-a687-fb1b89e71856" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortMember_019ecf33-f542-7f0d-be62-12df01e76b47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PositionDomain_019ecf33-f542-71bc-a687-fb1b89e71856" xlink:to="loc_us-gaap_ShortMember_019ecf33-f542-7f0d-be62-12df01e76b47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongMember_019ecf33-f542-7898-b7ca-d0acc79a25fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PositionDomain_019ecf33-f542-71bc-a687-fb1b89e71856" xlink:to="loc_us-gaap_LongMember_019ecf33-f542-7898-b7ca-d0acc79a25fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-75ad-bc79-beb04a02f9c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-7695-b2b0-0cde960cdca1" xlink:to="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-75ad-bc79-beb04a02f9c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-75ad-bc79-beb04a02f9c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-75ad-bc79-beb04a02f9c6" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-75ad-bc79-beb04a02f9c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7846-8ffa-39801bab23ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-75ad-bc79-beb04a02f9c6" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7846-8ffa-39801bab23ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019ecf33-f542-79b4-843e-7233e0b87a44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7846-8ffa-39801bab23ac" xlink:to="loc_us-gaap_NondesignatedMember_019ecf33-f542-79b4-843e-7233e0b87a44" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019ecf33-f542-7b54-8673-ccf2f7b82881" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-76cf-ab8e-5321230611e4" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019ecf33-f542-7b54-8673-ccf2f7b82881" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCIALINSTRUMENTSNarrativeDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCIALINSTRUMENTSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCIALINSTRUMENTSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-7616-b99a-62fa08b4598b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-745d-bbba-1f3e0e2ca26c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-7616-b99a-62fa08b4598b" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-745d-bbba-1f3e0e2ca26c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7756-80e8-d3540abf260b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-745d-bbba-1f3e0e2ca26c" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7756-80e8-d3540abf260b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7756-80e8-d3540abf260b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7756-80e8-d3540abf260b" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7756-80e8-d3540abf260b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-727e-b957-30200ee5cb90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7756-80e8-d3540abf260b" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-727e-b957-30200ee5cb90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7a7f-9c3e-6ad39238251a" xlink:href="cstm-20260630.xsd#cstm_ForeignExchangeContractCommercialDerivativesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-727e-b957-30200ee5cb90" xlink:to="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7a7f-9c3e-6ad39238251a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ForeignExchangeContractNetDebtDerivativesMember_019ecf33-f542-7944-b7fe-8ba66ee2d903" xlink:href="cstm-20260630.xsd#cstm_ForeignExchangeContractNetDebtDerivativesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-727e-b957-30200ee5cb90" xlink:to="loc_cstm_ForeignExchangeContractNetDebtDerivativesMember_019ecf33-f542-7944-b7fe-8ba66ee2d903" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7bc5-999c-4dde8057bba1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-745d-bbba-1f3e0e2ca26c" xlink:to="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7bc5-999c-4dde8057bba1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7bc5-999c-4dde8057bba1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7bc5-999c-4dde8057bba1" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7bc5-999c-4dde8057bba1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-773a-8902-764c3b7a7dfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7bc5-999c-4dde8057bba1" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-773a-8902-764c3b7a7dfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-793a-9d8b-8f491d1ddae1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-773a-8902-764c3b7a7dfc" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-793a-9d8b-8f491d1ddae1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019ecf33-f542-7ce4-a315-fc4180eb6e53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-773a-8902-764c3b7a7dfc" xlink:to="loc_us-gaap_NondesignatedMember_019ecf33-f542-7ce4-a315-fc4180eb6e53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019ecf33-f542-72a3-b173-b2b2919b1b4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-7616-b99a-62fa08b4598b" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019ecf33-f542-72a3-b173-b2b2919b1b4d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofderivativesinstrumentsperformanceDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#FINANCIALINSTRUMENTSScheduleofderivativesinstrumentsperformanceDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/FINANCIALINSTRUMENTSScheduleofderivativesinstrumentsperformanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-77bc-a7cb-bcdc14227b0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-77bc-a7cb-bcdc14227b0f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7b56-8a71-38c73ffa1e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-77bc-a7cb-bcdc14227b0f" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7b56-8a71-38c73ffa1e4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7b56-8a71-38c73ffa1e4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7b56-8a71-38c73ffa1e4a" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7b56-8a71-38c73ffa1e4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7131-8508-fe8d4d16dd64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ecf33-f542-7b56-8a71-38c73ffa1e4a" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7131-8508-fe8d4d16dd64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7c1f-bb2f-33e3ef64e77f" xlink:href="cstm-20260630.xsd#cstm_ForeignExchangeContractCommercialDerivativesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7131-8508-fe8d4d16dd64" xlink:to="loc_cstm_ForeignExchangeContractCommercialDerivativesMember_019ecf33-f542-7c1f-bb2f-33e3ef64e77f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ForeignExchangeContractNetDebtDerivativesMember_019ecf33-f542-75a1-9edd-5085c761f10b" xlink:href="cstm-20260630.xsd#cstm_ForeignExchangeContractNetDebtDerivativesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7131-8508-fe8d4d16dd64" xlink:to="loc_cstm_ForeignExchangeContractNetDebtDerivativesMember_019ecf33-f542-75a1-9edd-5085c761f10b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommodityMember_019ecf33-f542-7b06-b5a3-2742b871d1c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommodityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ecf33-f542-7131-8508-fe8d4d16dd64" xlink:to="loc_us-gaap_CommodityMember_019ecf33-f542-7b06-b5a3-2742b871d1c5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7c82-885f-2b876ff3114f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ecf33-f542-77bc-a7cb-bcdc14227b0f" xlink:to="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7c82-885f-2b876ff3114f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7c82-885f-2b876ff3114f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7c82-885f-2b876ff3114f" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7c82-885f-2b876ff3114f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7731-ba1a-ec4e2e9cd66e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ecf33-f542-7c82-885f-2b876ff3114f" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7731-ba1a-ec4e2e9cd66e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019ecf33-f542-7414-ac19-0b96a3d05aba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7731-ba1a-ec4e2e9cd66e" xlink:to="loc_us-gaap_NondesignatedMember_019ecf33-f542-7414-ac19-0b96a3d05aba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-7739-816d-ff3d255d4b1f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ecf33-f542-7731-ba1a-ec4e2e9cd66e" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ecf33-f542-7739-816d-ff3d255d4b1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_RealizedGainLossOnDerivativesNet_019ecf33-f542-7d5b-bb11-ebd36c99e127" xlink:href="cstm-20260630.xsd#cstm_RealizedGainLossOnDerivativesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_cstm_RealizedGainLossOnDerivativesNet_019ecf33-f542-7d5b-bb11-ebd36c99e127" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ecf33-f542-7507-81df-b45432dc4773" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnrealizedGainLossOnDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ecf33-f542-7507-81df-b45432dc4773" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax_019ecf33-f542-75ff-b04c-a8601118f7ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax_019ecf33-f542-75ff-b04c-a8601118f7ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019ecf33-f542-7343-8824-871f653f3664" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019ecf33-f542-7343-8824-871f653f3664" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized_019ecf33-f542-7602-b271-1096bf4624ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossRealized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized_019ecf33-f542-7602-b271-1096bf4624ae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_019ecf33-f542-73b3-805f-7389eeb2d179" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_019ecf33-f542-73b3-805f-7389eeb2d179" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_019ecf33-f542-7d3c-bdc2-678cc17eccdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ecf33-f542-71dc-9008-690a2f5f5f15" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_019ecf33-f542-7d3c-bdc2-678cc17eccdd" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/PENSIONANDOTHERPOSTEMPLOYMENTBENEFITOBLIGATIONSDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#PENSIONANDOTHERPOSTEMPLOYMENTBENEFITOBLIGATIONSDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/PENSIONANDOTHERPOSTEMPLOYMENTBENEFITOBLIGATIONSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019ecf33-f542-718a-aa7d-e763b3c527d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019ecf33-f542-718a-aa7d-e763b3c527d4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_019ecf33-f542-748c-a154-367b67fe3b62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019ecf33-f542-718a-aa7d-e763b3c527d4" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_019ecf33-f542-748c-a154-367b67fe3b62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-748c-a154-367b67fe3b62_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019ecf33-f542-748c-a154-367b67fe3b62" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-748c-a154-367b67fe3b62_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-711b-95eb-624d2cb64712" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019ecf33-f542-748c-a154-367b67fe3b62" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-711b-95eb-624d2cb64712" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_019ecf33-f542-73e7-92b9-7af66d83d4af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-711b-95eb-624d2cb64712" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_019ecf33-f542-73e7-92b9-7af66d83d4af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_OtherPostEmploymentBenefitsAndOtherLongTermBenefitMember_019ecf33-f542-7934-a216-55d09dda2ceb" xlink:href="cstm-20260630.xsd#cstm_OtherPostEmploymentBenefitsAndOtherLongTermBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019ecf33-f542-711b-95eb-624d2cb64712" xlink:to="loc_cstm_OtherPostEmploymentBenefitsAndOtherLongTermBenefitMember_019ecf33-f542-7934-a216-55d09dda2ceb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_019ecf33-f542-7157-aada-3688ada26387" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_019ecf33-f542-7157-aada-3688ada26387" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_019ecf33-f542-74e5-94b3-a83f06754161" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_019ecf33-f542-74e5-94b3-a83f06754161" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_019ecf33-f542-7059-a6ba-779b4c012237" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_019ecf33-f542-7059-a6ba-779b4c012237" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit_019f65f1-389e-7d60-a2b1-069c2cda9ef9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit_019f65f1-389e-7d60-a2b1-069c2cda9ef9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_019ecf33-f542-7f01-bb33-2c14328e0404" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_019ecf33-f542-7f01-bb33-2c14328e0404" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_019ecfc0-d9f1-7a9a-9297-482c5ac19f76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_019ecfc0-d9f1-7a9a-9297-482c5ac19f76" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_019ecf33-f542-7c1f-841d-d097cf51e87e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019ecf33-f542-7fa8-b038-ae860b8c585c" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_019ecf33-f542-7c1f-841d-d097cf51e87e" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/ACCUMULATEDOTHERCOMPREHENSIVEINCOMEDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#ACCUMULATEDOTHERCOMPREHENSIVEINCOMEDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/ACCUMULATEDOTHERCOMPREHENSIVEINCOMEDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ecf33-f542-753d-84e4-33e17c7c278b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ecf33-f542-7a37-8f04-a352d7818585" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ecf33-f542-753d-84e4-33e17c7c278b" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ecf33-f542-7a37-8f04-a352d7818585" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f542-7101-8300-6661964d1459" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ecf33-f542-7a37-8f04-a352d7818585" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f542-7101-8300-6661964d1459" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ecf33-f542-7101-8300-6661964d1459_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f542-7101-8300-6661964d1459" xlink:to="loc_us-gaap_EquityComponentDomain_019ecf33-f542-7101-8300-6661964d1459_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ecf33-f542-7745-acae-812c8297fd4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ecf33-f542-7101-8300-6661964d1459" xlink:to="loc_us-gaap_EquityComponentDomain_019ecf33-f542-7745-acae-812c8297fd4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f542-7ecb-93dd-f555b266b148" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ecf33-f542-7745-acae-812c8297fd4e" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f542-7ecb-93dd-f555b266b148" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_019ecf33-f542-7c8e-9513-b060e4b164f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f542-7ecb-93dd-f555b266b148" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_019ecf33-f542-7c8e-9513-b060e4b164f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ecf33-f542-7db7-9f66-9dc5d3a2d01f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f542-7ecb-93dd-f555b266b148" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ecf33-f542-7db7-9f66-9dc5d3a2d01f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019ecf33-f542-776d-9797-6504a992cf36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ecf33-f542-7ecb-93dd-f555b266b148" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_019ecf33-f542-776d-9797-6504a992cf36" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ecf33-f542-753d-84e4-33e17c7c278b" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f542-7b5a-9937-3c74a22dff8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f542-7b5a-9937-3c74a22dff8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_019ecf33-f542-77eb-8b72-f30f484f9ed2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_019ecf33-f542-77eb-8b72-f30f484f9ed2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_019ecf33-f542-73ce-898f-d82642777492" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_019ecf33-f542-73ce-898f-d82642777492" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ReclassificationFromAccumulatedOtherComprehensiveIncomeToRetainedEarningsCurrentPeriodNetOfTax_019ecf33-f542-73ea-95c3-b2a66899c87a" xlink:href="cstm-20260630.xsd#cstm_ReclassificationFromAccumulatedOtherComprehensiveIncomeToRetainedEarningsCurrentPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ecf33-f542-7900-b4e6-37a6477b1c2f" xlink:to="loc_cstm_ReclassificationFromAccumulatedOtherComprehensiveIncomeToRetainedEarningsCurrentPeriodNetOfTax_019ecf33-f542-73ea-95c3-b2a66899c87a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019ecf33-f542-764a-822a-34a7fedbc189" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SHAREBASEDCOMPENSATIONNarrativeDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SHAREBASEDCOMPENSATIONNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SHAREBASEDCOMPENSATIONNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-74ee-b2f9-e2d5e0e18b3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-74ee-b2f9-e2d5e0e18b3e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7bfd-8ea2-a8852ff48c72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-74ee-b2f9-e2d5e0e18b3e" xlink:to="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7bfd-8ea2-a8852ff48c72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7bfd-8ea2-a8852ff48c72_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7bfd-8ea2-a8852ff48c72" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7bfd-8ea2-a8852ff48c72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7535-8cb4-6e5c6d6473cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7bfd-8ea2-a8852ff48c72" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7535-8cb4-6e5c6d6473cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019ecf33-f542-77f7-9ba4-8f171312f1f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7535-8cb4-6e5c6d6473cd" xlink:to="loc_us-gaap_PerformanceSharesMember_019ecf33-f542-77f7-9ba4-8f171312f1f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019ecf33-f542-7c61-8cef-13e3df709a8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7535-8cb4-6e5c6d6473cd" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019ecf33-f542-7c61-8cef-13e3df709a8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ecf33-f542-784c-9f2d-9c92ad143d96" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-74ee-b2f9-e2d5e0e18b3e" xlink:to="loc_srt_RangeAxis_019ecf33-f542-784c-9f2d-9c92ad143d96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ecf33-f542-784c-9f2d-9c92ad143d96_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ecf33-f542-784c-9f2d-9c92ad143d96" xlink:to="loc_srt_RangeMember_019ecf33-f542-784c-9f2d-9c92ad143d96_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ecf33-f542-716d-bc89-5edc52d54c28" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ecf33-f542-784c-9f2d-9c92ad143d96" xlink:to="loc_srt_RangeMember_019ecf33-f542-716d-bc89-5edc52d54c28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ecf33-f542-789b-bcd8-45ab5c0c0ee1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ecf33-f542-716d-bc89-5edc52d54c28" xlink:to="loc_srt_MinimumMember_019ecf33-f542-789b-bcd8-45ab5c0c0ee1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ecf33-f542-7c09-b7d9-90b29706c506" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ecf33-f542-716d-bc89-5edc52d54c28" xlink:to="loc_srt_MaximumMember_019ecf33-f542-7c09-b7d9-90b29706c506" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_019ecf33-f542-766b-a12b-18e30aa69bf0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-74ee-b2f9-e2d5e0e18b3e" xlink:to="loc_us-gaap_GranteeStatusAxis_019ecf33-f542-766b-a12b-18e30aa69bf0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019ecf33-f542-766b-a12b-18e30aa69bf0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_019ecf33-f542-766b-a12b-18e30aa69bf0" xlink:to="loc_us-gaap_GranteeStatusDomain_019ecf33-f542-766b-a12b-18e30aa69bf0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019ecf33-f542-7b40-98f2-67520ac6957e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_019ecf33-f542-766b-a12b-18e30aa69bf0" xlink:to="loc_us-gaap_GranteeStatusDomain_019ecf33-f542-7b40-98f2-67520ac6957e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019ecf33-f542-7ad5-a887-78555592db36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedPaymentArrangementEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019ecf33-f542-7b40-98f2-67520ac6957e" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019ecf33-f542-7ad5-a887-78555592db36" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019ecf33-f542-7968-bf3e-5b847d3fe47d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019ecf33-f542-7968-bf3e-5b847d3fe47d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019ecf33-f542-7462-bbfa-3bb35c3871b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019ecf33-f542-7462-bbfa-3bb35c3871b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_PercentageOfShareBasedPaymentsVestingMultiplier_019ecf33-f542-7afb-acda-efb2e98d5fd4" xlink:href="cstm-20260630.xsd#cstm_PercentageOfShareBasedPaymentsVestingMultiplier"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_cstm_PercentageOfShareBasedPaymentsVestingMultiplier_019ecf33-f542-7afb-acda-efb2e98d5fd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ecf33-f542-7d43-ae7a-ed17dbea1078" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ecf33-f542-7d43-ae7a-ed17dbea1078" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ecf33-f542-7e33-ab79-6d79846d67ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ecf33-f542-7e33-ab79-6d79846d67ff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019ecf33-f542-7f16-b367-a4c626f1804b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019ecf33-f542-7f16-b367-a4c626f1804b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019ecf33-f542-73ff-9d30-34dcb74b580a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019ecf33-f542-73ff-9d30-34dcb74b580a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019ecf33-f542-7973-8f7d-4cd8a0ca9ad0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7a05-a28e-59351546cdc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019ecf33-f542-7973-8f7d-4cd8a0ca9ad0" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SHAREBASEDCOMPENSATIONScheduleofvaluationmodelusedforthePSUsgrantedDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SHAREBASEDCOMPENSATIONScheduleofvaluationmodelusedforthePSUsgrantedDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SHAREBASEDCOMPENSATIONScheduleofvaluationmodelusedforthePSUsgrantedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-715f-8c35-fabdd116259f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-715f-8c35-fabdd116259f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7f1a-b7f9-0752522978ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ecf33-f542-715f-8c35-fabdd116259f" xlink:to="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7f1a-b7f9-0752522978ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7f1a-b7f9-0752522978ef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7f1a-b7f9-0752522978ef" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7f1a-b7f9-0752522978ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7507-b4cc-5032407ca84d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ecf33-f542-7f1a-b7f9-0752522978ef" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7507-b4cc-5032407ca84d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019ecf33-f542-7d43-ada2-5a2ccbd958d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ecf33-f542-7507-b4cc-5032407ca84d" xlink:to="loc_us-gaap_PerformanceSharesMember_019ecf33-f542-7d43-ada2-5a2ccbd958d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ecf33-f542-7bdd-8c97-5b0b04c73fca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ecf33-f542-7bdd-8c97-5b0b04c73fca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_019ecf33-f542-7213-b157-cdd647d9802e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_SharePrice_019ecf33-f542-7213-b157-cdd647d9802e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019ecf33-f542-7ad3-bccf-7004849e9221" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019ecf33-f542-7ad3-bccf-7004849e9221" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019ecf33-f542-7614-8ee4-aede89fefdbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019ecf33-f542-7614-8ee4-aede89fefdbf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019ecf33-f542-7e6e-baf1-af710db09d35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ecf33-f542-7240-b284-19fee49b170c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019ecf33-f542-7e6e-baf1-af710db09d35" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/ACQUISITIONANDDISPOSALOFSUBSIDIARIESDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#ACQUISITIONANDDISPOSALOFSUBSIDIARIESDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/ACQUISITIONANDDISPOSALOFSUBSIDIARIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019f65fb-5601-7fef-ba0d-6aa910366c7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019f65fb-5601-7b2f-8f25-d7b07da8aad7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019f65fb-5601-7fef-ba0d-6aa910366c7f" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019f65fb-5601-7b2f-8f25-d7b07da8aad7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019f65fd-0d65-7d21-999e-43904e43d209" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019f65fb-5601-7b2f-8f25-d7b07da8aad7" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019f65fd-0d65-7d21-999e-43904e43d209" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019f65fd-0d65-7d21-999e-43904e43d209_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019f65fd-0d65-7d21-999e-43904e43d209" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019f65fd-0d65-7d21-999e-43904e43d209_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019f65fd-0d65-7ab5-a585-62b46e8617a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019f65fd-0d65-7d21-999e-43904e43d209" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019f65fd-0d65-7ab5-a585-62b46e8617a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ChangchunEngleyAutomobilePartsCoLtdChangchunMember_019f65fd-cb1b-7be3-b66c-21b1a4b0f18c" xlink:href="cstm-20260630.xsd#cstm_ChangchunEngleyAutomobilePartsCoLtdChangchunMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019f65fd-0d65-7ab5-a585-62b46e8617a9" xlink:to="loc_cstm_ChangchunEngleyAutomobilePartsCoLtdChangchunMember_019f65fd-cb1b-7be3-b66c-21b1a4b0f18c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_019f65fe-fbfc-7d39-8590-96e5a8b1c550" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019f65fb-5601-7b2f-8f25-d7b07da8aad7" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_019f65fe-fbfc-7d39-8590-96e5a8b1c550" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019f65fe-fbfc-7d39-8590-96e5a8b1c550_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019f65fe-fbfc-7d39-8590-96e5a8b1c550" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019f65fe-fbfc-7d39-8590-96e5a8b1c550_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019f65fe-fbfc-72e8-98c4-2621dd5a475b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019f65fe-fbfc-7d39-8590-96e5a8b1c550" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019f65fe-fbfc-72e8-98c4-2621dd5a475b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember_019f65ff-a87f-7cbc-95f5-55cca003c1cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_019f65fe-fbfc-72e8-98c4-2621dd5a475b" xlink:to="loc_us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember_019f65ff-a87f-7cbc-95f5-55cca003c1cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_DisposalGroupIncludingDiscontinuedOperationOwnershipPercentageInDisposedAsset_019f6604-db3f-70b4-8c46-fae588d0584e" xlink:href="cstm-20260630.xsd#cstm_DisposalGroupIncludingDiscontinuedOperationOwnershipPercentageInDisposedAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019f65fb-5601-7fef-ba0d-6aa910366c7f" xlink:to="loc_cstm_DisposalGroupIncludingDiscontinuedOperationOwnershipPercentageInDisposedAsset_019f6604-db3f-70b4-8c46-fae588d0584e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_019f6620-b276-76e8-8833-af8cdf9364da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromDivestitureOfBusinesses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019f65fb-5601-7fef-ba0d-6aa910366c7f" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_019f6620-b276-76e8-8833-af8cdf9364da" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.constellium.com/role/SUBSEQUENTEVENTSDetails" xlink:type="simple" xlink:href="cstm-20260630.xsd#SUBSEQUENTEVENTSDetails"/>
  <link:definitionLink xlink:role="http://www.constellium.com/role/SUBSEQUENTEVENTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019f7e8c-7a29-75d0-8e56-517fd4df3a60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019f7e8c-7a29-7f81-a34e-200074e64c1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019f7e8c-7a29-75d0-8e56-517fd4df3a60" xlink:to="loc_us-gaap_SubsequentEventTable_019f7e8c-7a29-7f81-a34e-200074e64c1a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019f7e8c-7a29-7f81-a34e-200074e64c1a" xlink:to="loc_us-gaap_DebtInstrumentAxis_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f7e8d-1491-7b4c-8337-31deebaefb80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f7e8d-1491-7aaa-a2a2-d7c182f0c1b5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f7e8d-1491-7b4c-8337-31deebaefb80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019f8f19-cf3a-7440-a6bd-865c59690e45" xlink:href="cstm-20260630.xsd#cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f7e8d-1491-7b4c-8337-31deebaefb80" xlink:to="loc_cstm_ConstelliumSESeniorUnsecuredNotesIssuedJune2020DueJune2028325MillionMember_019f8f19-cf3a-7440-a6bd-865c59690e45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f7e8d-d2a4-7bad-b1da-ad36c154e01c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019f7e8c-7a29-7f81-a34e-200074e64c1a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f7e8d-d2a4-7bad-b1da-ad36c154e01c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f7e8d-d2a4-7bad-b1da-ad36c154e01c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f7e8d-d2a4-7bad-b1da-ad36c154e01c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f7e8d-d2a4-7bad-b1da-ad36c154e01c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f7e8d-d2a4-70c6-acb6-74cb9c9b6a0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f7e8d-d2a4-7bad-b1da-ad36c154e01c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f7e8d-d2a4-70c6-acb6-74cb9c9b6a0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019f8f1a-173c-7db7-8bef-9fcdeafe44cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f7e8d-d2a4-70c6-acb6-74cb9c9b6a0c" xlink:to="loc_us-gaap_SeniorNotesMember_019f8f1a-173c-7db7-8bef-9fcdeafe44cb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019f7e8e-5678-7166-84f3-511648340a04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019f7e8c-7a29-7f81-a34e-200074e64c1a" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019f7e8e-5678-7166-84f3-511648340a04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f7e8e-5678-7166-84f3-511648340a04_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f7e8e-5678-7166-84f3-511648340a04" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f7e8e-5678-7166-84f3-511648340a04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f7e8e-5678-7aa3-873b-d810f5626828" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f7e8e-5678-7166-84f3-511648340a04" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f7e8e-5678-7aa3-873b-d810f5626828" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019f7e8e-bf40-7225-9702-18afe5f36a8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019f7e8e-5678-7aa3-873b-d810f5626828" xlink:to="loc_us-gaap_SubsequentEventMember_019f7e8e-bf40-7225-9702-18afe5f36a8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_019f9357-75c2-7868-a286-2f51d47381cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019f7e8c-7a29-75d0-8e56-517fd4df3a60" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_019f9357-75c2-7868-a286-2f51d47381cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019f7e8c-7a29-7649-aca4-6a5c1879bd26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019f7e8c-7a29-75d0-8e56-517fd4df3a60" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019f7e8c-7a29-7649-aca4-6a5c1879bd26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f7e93-496d-7ed9-ba18-f6c729498234" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019f7e8c-7a29-75d0-8e56-517fd4df3a60" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f7e93-496d-7ed9-ba18-f6c729498234" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
