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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-03-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001676639</rptOwnerCik>
            <rptOwnerName>Johnson Brian Joseph</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>SVP, Store Development</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-03-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>8000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>44.39</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18817</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-03-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>169.02</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16817</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-03-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>167.80</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11017</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-03-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>166.37</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10817</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2450</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Voting and tender rights under 401k plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Option - right to buy</value>
                <footnoteId id="F4"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>44.39</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2020-03-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>8000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2014-06-23</value>
            </exerciseDate>
            <expirationDate>
                <value>2021-06-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>8000.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3000</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="F6"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F5"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1141.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1141</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F7"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1238.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1238</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F8"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F8"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>919.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>919</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $168.44 to $169.09. Mr. Johnson undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F2">The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $167.37 to $168.23. Mr. Johnson undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F3">Allocated to Mr. Johnson's 401k plan account as of March 18, 2020. Does not include any shares allocated by the plan trustee after that date.</footnote>
        <footnote id="F4">Pursuant to terms and conditions of 2009 Stock Incentive Plan.</footnote>
        <footnote id="F5">Pursuant to terms and conditions of 2009 Stock Incentive Plan.  This award will vest in full on June 15, 2020. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2020, 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="F6">Each restricted stock unit represents the right to receive, following vesting, one share of Common Stock.</footnote>
        <footnote id="F7">Pursuant to terms and conditions of 2009 Stock Incentive Plan.  This award will vest in full on June 15, 2021. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2021, 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="F8">Pursuant to terms and conditions of 2018 Stock Incentive Plan.  This award will vest in full on June 15, 2022. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2022, 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 March 3, 2017</signatureName>
        <signatureDate>2020-03-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2020-03-18
0
0000726958
CASEYS GENERAL STORES INC
CASY
0001676639
Johnson Brian Joseph
ONE SE CONVENIENCE BLVD.
ANKENY
IA
50021
0
1
0
0
SVP, Store Development
Common Stock
2020-03-18
4
M
0
8000
44.39
A
18817
D
Common Stock
2020-03-18
4
S
0
2000
169.02
D
16817
D
Common Stock
2020-03-18
4
S
0
5800
167.80
D
11017
D
Common Stock
2020-03-18
4
S
0
200
166.37
D
10817
D
Common Stock
2450
I
Voting and tender rights under 401k plan
Option - right to buy
44.39
2020-03-18
4
M
0
8000
0
D
2014-06-23
2021-06-23
Common Stock
8000.0
3000
D
Restricted stock units
Common Stock
1141.0
1141
D
Restricted stock units
Common Stock
1238.0
1238
D
Restricted stock units
Common Stock
919.0
919
D
The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $168.44 to $169.09. Mr. Johnson undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $167.37 to $168.23. Mr. Johnson undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
Allocated to Mr. Johnson's 401k plan account as of March 18, 2020. Does not include any shares allocated by the plan trustee after that date.
Pursuant to terms and conditions of 2009 Stock Incentive Plan.
Pursuant to terms and conditions of 2009 Stock Incentive Plan.  This award will vest in full on June 15, 2020. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2020, 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 2009 Stock Incentive Plan.  This award will vest in full on June 15, 2021. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2021, 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 full on June 15, 2022. Not included in the reported award amount is a target amount of performance-based restricted stock units that will vest on June 15, 2022, 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 March 3, 2017
2020-03-20