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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-06-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001040971</issuerCik>
        <issuerName>SL GREEN REALTY CORP</issuerName>
        <issuerTradingSymbol>SLG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001216194</rptOwnerCik>
            <rptOwnerName>HOLLIDAY MARC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SL GREEN REALTY CORP.</rptOwnerStreet1>
            <rptOwnerStreet2>420 LEXINGTON AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10170</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Executive Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Class O LTIP Units</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>99.86</value>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-06-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>52500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2017-06-17</value>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-06-17</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>52500</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>52500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Class O LTIP Units</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>99.86</value>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-06-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>52500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2017-06-17</value>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-06-17</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>52500</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>52500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents Class O LTIP Units granted in connection with provisions contained in an employment agreement dated as of February 10, 2016 between the reporting person and the Issuer, as previously disclosed.</footnote>
        <footnote id="F2">The Class O LTIP Units, once vested, may be converted, at the election of the holder, into a number of Class A Units of limited partnership interest in SL Green Operating Partnership, L.P. (&quot;Common Units&quot;) determined by the increase in value of a share of the Issuer's Common Stock at the time of conversion over $99.86, which was the fair market value of a share of the Issuer's Common Stock at the time of grant pursuant to the SL Green Realty Corp. Fourth Amended and Restated 2005 Stock Option and Incentive Plan (the &quot;Plan&quot;).</footnote>
        <footnote id="F3">Each Common Unit acquired upon conversion of vested Class O LTIP Units may be presented for redemption, at the election of the holder, for cash equal to the then fair market value of a share of the Issuer's Common Stock, except that the Issuer may, at its election, acquire each Common Unit so presented for one share of Common Stock. Class O LTIP Units and the Common Units into which they may be converted generally may not be disposed of without the consent of the Issuer until two years from the date of the grant of the Class O LTIP Units.</footnote>
        <footnote id="F4">Represents the fair market value of a share of the Issuer's Common Stock at the time of grant, as determined pursuant to the Plan.</footnote>
        <footnote id="F5">The Class O LTIP Units vest on June 17, 2017, subject to the reporting person's continued employment through such date. The rights to convert these Class O LTIP Units into Common Units have a maximum term of ten years. The rights to redeem Common Units do not have expiration dates.</footnote>
        <footnote id="F6">The Class O LTIP Units vest on June 17, 2017, subject to the reporting person's continued employment through such date. The rights to convert these Class O LTIP Units into Common Units have a maximum term of five years. The rights to redeem Common Units do not have expiration dates.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Marc Holliday</signatureName>
        <signatureDate>2016-06-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
