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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-06-14</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001362925</issuerCik>
        <issuerName>Planet Payment Inc</issuerName>
        <issuerTradingSymbol>PLPM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001562400</rptOwnerCik>
            <rptOwnerName>Kaiden Jonathan</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PLANET PAYMENT, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>670 LONG BEACH BLVD.</rptOwnerStreet2>
            <rptOwnerCity>LONG BEACH</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>11561</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</value>
            </securityTitle>
            <transactionDate>
                <value>2016-06-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>14815</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.05</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>108610</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>42550</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This is a restricted stock grant under the Issuer's 2012 Equity Incentive Plan, and will be 100% vested on June 14, 2017, subject to earlier acceleration in full upon a change in control, and provided that the Reporting Person is still providing services to the Issuer as of such date.</footnote>
        <footnote id="F2">Jonathan Kaiden is the Managing Member of Kaiden Investment Group (&quot;KIG&quot;). As such, Mr. Kaiden may be deemed to beneficially own the shares of CommonStock owned by KIG. Mr. Kaiden disclaims beneficial ownership of the shares of Common Stock beneficially owned by KIG except to the extent of his pecuniaryinterest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>David R. Fishkin, as attorney-in-fact</signatureName>
        <signatureDate>2016-06-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
