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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-05-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001095651</issuerCik>
        <issuerName>ISTAR INC.</issuerName>
        <issuerTradingSymbol>STAR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001044258</rptOwnerCik>
            <rptOwnerName>RIDINGS BARRY W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ISTAR</rptOwnerStreet1>
            <rptOwnerStreet2>1114 AVENUE OF THE AMERICAS 39TH FLR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.001 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2017-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10146</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>58037</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Common Stock Equivalents</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6670</value>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6670</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On May 16, 2017, the Reporting Person, Barry Ridings, was awarded a total of 10,146 restricted shares of Common Stock of iStar Inc. (NYSE:STAR) in accordance with the provisions of the iStar  Inc. Non-Employee Directors' Deferral Plan (Plan), which are owned directly. These shares are restricted and may not be sold or otherwise transferred until they vest on May 16, 2018.</footnote>
        <footnote id="F2">Following this transaction, the Reporting Person is the beneficial owner of 58,037 shares of iStar Common Stock, which are owned directly.</footnote>
        <footnote id="F3">The Reporting Person also holds a total of 6,670 Common Stock Equivalents (CSEs) awarded pursuant to the Plan, all of which are vested. Under the Plan, on the regular distribution date, vested CSEs will be settled by the transfer of shares of iStar Common Stock to the participant. The &quot;regular distribution date&quot; for distributions to Plan participants is the earlier of: (1) January 1 on or next following the earlier of (i) the date the participant ceases to be a non-employee director; and (ii) the date of the participant's death; and (2) a change of control (as defined in the Plan). A participant, under certain limited circumstances, is permitted to elect to receive distributions at times other than the regular distribution date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>S/ BARRY W RIDINGS</signatureName>
        <signatureDate>2017-05-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
