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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-05-09</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001547903</issuerCik>
        <issuerName>NMI Holdings, Inc.</issuerName>
        <issuerTradingSymbol>NMIH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001591062</rptOwnerCik>
            <rptOwnerName>Osmon John Brandon</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O NMI HOLDINGS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2100 POWELL STREET, 12TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>EMERYVILLE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94608</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Former Dir.; Former 10% Owner</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001420192</rptOwnerCik>
            <rptOwnerName>HAYMAN CAPITAL MANAGEMENT, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2101 CEDAR SPRINGS ROAD, SUITE 1400</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>DALLAS</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>75201</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Former Dir.; Former 10% Owner</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001420193</rptOwnerCik>
            <rptOwnerName>Bass J Kyle</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O HAYMAN INVESTMENTS, L.L.C.</rptOwnerStreet1>
            <rptOwnerStreet2>2101 CEDAR SPRINGS ROAD, SUITE 1400</rptOwnerStreet2>
            <rptOwnerCity>DALLAS</rptOwnerCity>
            <rptOwnerState>X1</rptOwnerState>
            <rptOwnerZipCode>75201</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001420196</rptOwnerCik>
            <rptOwnerName>Hayman Investments LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2101 CEDAR SPRINGS ROAD, SUITE 1400</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>DALLAS</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>75201</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class A Common Shares, $0.01 par value per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>39832</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Shares, $0.01 par value per share</value>
            </securityTitle>
            <transactionDate>
                <value>2016-05-09</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1750000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.30</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5621500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Shares, $0.01 par value per share</value>
            </securityTitle>
            <transactionDate>
                <value>2016-05-10</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>312800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.3241</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5308700</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Mr. Osmon is a managing director of Hayman Capital Management, L.P. (&quot;Hayman Capital Management&quot;). Mr. Osmon ceased to be a director of the Issuer as of May 10, 2016. Hayman Capital Management acts as an investment adviser to, and manages investment and trading accounts of, other persons, including Hayman Capital Master Fund, L.P. (&quot;HCMF&quot;). Hayman Investments, L.L.C. (&quot;Hayman Investments&quot;) is the general partner of Hayman Capital Management. Mr. Bass is the managing member of Hayman Investments. In the foregoing capacities, Hayman Capital Management, Hayman Investments and Mr. Bass may be deemed to beneficially own securities beneficially owned by Hayman Capital Management. In connection with Mr. Osmon's employment by Hayman Capital Management, HCMF may be deemed to beneficially own securities of the Issuer beneficially owned by Mr. Osmon.</footnote>
        <footnote id="F2">Each of the Reporting Persons disclaims beneficial ownership of the securities reported herein except to the extent of his or its pecuniary interest in such securities, and the inclusion of such securities in this report shall not be deemed an admission of beneficial ownership for purposes of Section 16 or for any other purpose.</footnote>
        <footnote id="F3">Reflects the weighted average price of shares sold in multiple transactions at prices ranging from $6.314 to $6.3677.  The Reporting Persons undertake to provide to the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth above.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Brandon Osmon</signatureName>
        <signatureDate>2016-05-10</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Hayman Capital Management, L.P., by Hayman Investments, L.L.C., its general partner, by J Kyle Bass, Managing Member</signatureName>
        <signatureDate>2016-05-10</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Hayman Investments L.L.C., by J Kyle Bass, Managing Member</signatureName>
        <signatureDate>2016-05-10</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ J Kyle Bass</signatureName>
        <signatureDate>2016-05-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
