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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-01-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001902016</rptOwnerCik>
            <rptOwnerName>Kehle Shannon</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Chief People Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2174</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>53.41</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21929</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This amount represents shares automatically withheld upon the vesting of 3,647 restricted shares of common stock on January 1, 2025 and 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.</footnote>
        <footnote id="F2">Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on December 31, 2024.</footnote>
        <footnote id="F3">Includes 52 shares of common stock acquired through the Issuer's dividend reinvestment plan.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Bianca Martinez, by Power of Attorney</signatureName>
        <signatureDate>2025-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wk-form4_1735942300.xml", "description": "FORM 4", "secsgml_size_bytes": 3823}]
X0508
4
2025-01-01
0
0000726728
REALTY INCOME CORP
O
0001902016
Kehle Shannon
11995 EL CAMINO REAL
SAN DIEGO
CA
92130
0
1
0
0
EVP, Chief People Officer
0
Common Stock
2025-01-01
4
F
0
2174
53.41
D
21929
D
This amount represents shares automatically withheld upon the vesting of 3,647 restricted shares of common stock on January 1, 2025 and 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.
Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on December 31, 2024.
Includes 52 shares of common stock acquired through the Issuer's dividend reinvestment plan.
/s/ Bianca Martinez, by Power of Attorney
2025-01-03