<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-06-14</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001033012</issuerCik>
        <issuerName>FLAGSTAR BANCORP INC</issuerName>
        <issuerTradingSymbol>FBC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001455316</rptOwnerCik>
            <rptOwnerName>MP Thrift Investments L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>INTERTRUST CORP SERVICES DELAWARE LTD.</rptOwnerStreet1>
            <rptOwnerStreet2>200 BELLEVUE PARKWAY, SUITE 210</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19809</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-06-14</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>8000000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>33.8188</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>27600352</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares of Common Stock, par value $0.01 per share (the &quot;Common Stock&quot;), of Flagstar Bancorp, Inc. sold by MP Thrift Investments L.P. (&quot;MP Thrift&quot;) in a public offering pursuant to the Underwriting Agreement, dated June 11, 2018, with Sandler O'Neill &amp; Partners, L.P. and Keefe, Bruyette &amp; Woods, Inc. as underwriters. The offering closed on June 14, 2018. The shares of Common Stock were sold at a public offering price of $34.50 per share.</footnote>
        <footnote id="F2">MPGOP III Thrift AV-I L.P. (&quot;MPGOP&quot;), a Delaware limited partnership and MPGOP (Cayman) III Thrift AV-I L.P., a Cayman Islands exempted limited partnership (&quot;MPGOP Cayman&quot;, and together with MPGOP, the &quot;New Fund&quot;) together hold 100% of the membership interests in MP Thrift. MPGOP has a 77.05% interest in MP Thrift and MPGOP Cayman has a 22.95% interest in MP Thrift. MP (Thrift) Global Partners III LLC (&quot;MP LLC&quot;) is a limited liability company organized under the laws of Delaware. The principal business of MP LLC is to serve as general partner of each of MPGOP, MPGOP Cayman, MP Thrift, MP (Thrift) Global Opportunities Partners (Special) III LP (&quot;MPGOPS&quot;), and MP (Thrift) Global Opportunities Investments III LP (&quot;MPGOI&quot;).</footnote>
        <footnote id="F3">(continued from footnote 2) MP (Thrift) Asset Management LLC (&quot;MPAM&quot;) is a limited liability company organized under the laws of Delaware. MPAM is the managing member of MP LLC. MP (Thrift) LLC (&quot;MPT&quot;) is a limited liability company organized under the laws of Delaware. MPT is the managing member of MPAM. MPGOPS is a limited partnership organized under the laws of Delaware. MPGOPS is the holder of a 100% limited partnership interest in MPGOI. MPGOI is a limited partnership organized under the laws of Delaware. MPGOI is the holder of a 1% limited partnership interest in the New Fund. Along with MPT, MPAM, MP LLC, and MPGOPS, MPGOI comprises the Associates Fund.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Robert H. Weiss</signatureName>
        <signatureDate>2018-06-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
