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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-06-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726958</issuerCik>
        <issuerName>CASEYS GENERAL STORES INC</issuerName>
        <issuerTradingSymbol>CASY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001551683</rptOwnerCik>
            <rptOwnerName>Bramlage Stephen P JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE SE CONVENIENCE BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ANKENY</rptOwnerCity>
            <rptOwnerState>IA</rptOwnerState>
            <rptOwnerZipCode>50021</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2021-06-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2203</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2203</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2021-06-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1044</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>214.83</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1159</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2021-06-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2203</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2203.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4408</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F4"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2021-06-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1758</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1758.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1758</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F5"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F5"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1953.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1953</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Closing price of Casey's General Stores, Inc. Common Stock on June 2, 2021.</footnote>
        <footnote id="F2">Pursuant to terms and conditions of 2018 Stock Incentive Plan and in accordance with the terms and conditions of Mr. Bramlage's sign-on award of June 2, 2020, granted in conjunction with his commencement of employment as Chief Financial Officer. This award will vest at a rate of 1/3 of the initial award on each of the first three anniversaries of the award date. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2023, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.</footnote>
        <footnote id="F3">Each restricted stock unit represents the right to receive, following vesting, one share of Common Stock.</footnote>
        <footnote id="F4">Pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in equal installments on June 15, 2022, June 15, 2023 and June 15, 2024. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2024, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.</footnote>
        <footnote id="F5">Pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in equal installments on June 15, 2021, June 15, 2022 and June 15, 2023. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2023, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Scott Faber, under Power of Attorney dated June 1, 2020</signatureName>
        <signatureDate>2021-06-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2021-06-02
0
0000726958
CASEYS GENERAL STORES INC
CASY
0001551683
Bramlage Stephen P JR
ONE SE CONVENIENCE BLVD.
ANKENY
IA
50021
0
1
0
0
Chief Financial Officer
Common Stock
2021-06-02
4
M
0
2203
0
A
2203
D
Common Stock
2021-06-02
4
F
0
1044
214.83
D
1159
D
Restricted stock units
2021-06-02
4
M
0
2203
0
D
Common Stock
2203.0
4408
D
Restricted stock units
2021-06-02
4
A
0
1758
0
A
Common Stock
1758.0
1758
D
Restricted stock units
Common Stock
1953.0
1953
D
Closing price of Casey's General Stores, Inc. Common Stock on June 2, 2021.
Pursuant to terms and conditions of 2018 Stock Incentive Plan and in accordance with the terms and conditions of Mr. Bramlage's sign-on award of June 2, 2020, granted in conjunction with his commencement of employment as Chief Financial Officer. This award will vest at a rate of 1/3 of the initial award on each of the first three anniversaries of the award date. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2023, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.
Each restricted stock unit represents the right to receive, following vesting, one share of Common Stock.
Pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in equal installments on June 15, 2022, June 15, 2023 and June 15, 2024. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2024, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.
Pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in equal installments on June 15, 2021, June 15, 2022 and June 15, 2023. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2023, but which are subject to the satisfaction of certain performance criteria other than solely the price of Casey's Common Stock; the final amount of shares earned, if any, will be reported upon vest and satisfaction of those performance measures.
Scott Faber, under Power of Attorney dated June 1, 2020
2021-06-03