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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2024-02-12</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726728</issuerCik>
        <issuerName>REALTY INCOME CORP</issuerName>
        <issuerTradingSymbol>O</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001529329</rptOwnerCik>
            <rptOwnerName>Roy Sumit</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>11995 EL CAMINO REAL</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92130</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, CEO &amp; Director</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2024-02-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>82734</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
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                <sharesOwnedFollowingTransaction>
                    <value>300434</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
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                <directOrIndirectOwnership>
                    <value>D</value>
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            <securityTitle>
                <value>Common Stock</value>
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                <value>2024-02-12</value>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>22781</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>52.67</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
                    <value>277653</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2024-02-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>15814</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
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                <sharesOwnedFollowingTransaction>
                    <value>293467</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2024-02-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4354</value>
                    <footnoteId id="F5"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>52.67</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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                    <value>289113</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2024-02-12</value>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>33710</value>
                    <footnoteId id="F6"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>322823</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents performance shares awarded as a result of the Company having met certain performance criteria, pursuant to the reporting person's grant on February 17, 2021. Fifty percent of the performance shares immediately vested. The remaining fifty percent are subject to time vesting through January 1, 2025.</footnote>
        <footnote id="F2">Represents shares automatically withheld upon the issuance of 41,367 shares of common stock on February 12, 2024, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.</footnote>
        <footnote id="F3">Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on February 12, 2024.</footnote>
        <footnote id="F4">Represents performance shares awarded as a result of the Company having met certain performance criteria, pursuant to the reporting person's grant on November 15, 2021. Fifty percent of the performance shares immediately vested. The remaining fifty percent are subject to time vesting through December 31, 2024.</footnote>
        <footnote id="F5">Represents shares automatically withheld upon the issuance of 7,907 shares of common stock on February 12, 2024, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.</footnote>
        <footnote id="F6">Shares granted through an incentive plan; no consideration was paid. Shares vest ratably over four years.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Bianca Martinez, by Power of Attorney</signatureName>
        <signatureDate>2024-02-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0508
4
2024-02-12
0
0000726728
REALTY INCOME CORP
O
0001529329
Roy Sumit
11995 EL CAMINO REAL
SAN DIEGO
CA
92130
1
1
0
0
President, CEO & Director
0
Common Stock
2024-02-12
4
A
0
82734
0
A
300434
D
Common Stock
2024-02-12
4
F
0
22781
52.67
D
277653
D
Common Stock
2024-02-12
4
A
0
15814
0
A
293467
D
Common Stock
2024-02-12
4
F
0
4354
52.67
D
289113
D
Common Stock
2024-02-12
4
A
0
33710
0
A
322823
D
Represents performance shares awarded as a result of the Company having met certain performance criteria, pursuant to the reporting person's grant on February 17, 2021. Fifty percent of the performance shares immediately vested. The remaining fifty percent are subject to time vesting through January 1, 2025.
Represents shares automatically withheld upon the issuance of 41,367 shares of common stock on February 12, 2024, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.
Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on February 12, 2024.
Represents performance shares awarded as a result of the Company having met certain performance criteria, pursuant to the reporting person's grant on November 15, 2021. Fifty percent of the performance shares immediately vested. The remaining fifty percent are subject to time vesting through December 31, 2024.
Represents shares automatically withheld upon the issuance of 7,907 shares of common stock on February 12, 2024, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.
Shares granted through an incentive plan; no consideration was paid. Shares vest ratably over four years.
/s/ Bianca Martinez, by Power of Attorney
2024-02-14