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SEC EDGAR · Insider purchase

FUN: JAFFER REHAN reported buying shares

3 reported purchases of non-derivative securities. Code P includes open-market and private purchases; it does not establish motive.

Follow FUN
Became public
Nov 10, 2026, 5:43 PM EST
Event / period date
Nov 6, 2026
Collected
Nov 10, 2026, 5:46 PM EST

Why this is connected

This filing identifies Six Flags Entertainment Corporation/NEW as the issuer of the security. The company's SEC identifier connects it to FUN.

Open the original source ↗Follow company activity in FUN

Reported transactions

JAFFER REHAN (Director)

700,000 shares · Common Stock, par value $0.01 per share

Nov 6, 2026 · $10.00 per share

350,000 shares · Common Stock, par value $0.01 per share

Nov 9, 2026 · $10.86 per share

400,000 shares · Common Stock, par value $0.01 per share

Nov 10, 2026 · $10.32 per share

Value calculated from the reported shares and prices: $14,929,000.00.

Code P covers open-market and private purchases. Grants, option exercises, and tax withholding are excluded from purchase results. Read the source footnotes for ownership and pricing details.

Source evidence

Read the source excerpt or record
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-11-06</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001999001</issuerCik>
        <issuerName>Six Flags Entertainment Corporation/NEW</issuerName>
        <issuerTradingSymbol>FUN</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001364501</rptOwnerCik>
            <rptOwnerName>JAFFER REHAN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>C/O H PARTNERS MANAGEMENT LLC</rptOwnerStreet1>
            <rptOwnerStreet2>888 SEVENTH AVENUE, 29TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10019</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2026-11-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>700000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5600000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2026-11-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>350000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.86</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5950000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2026-11-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>400000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.32</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6350000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in column 4 is a weighted average price. These shares were purchased in multiple transactions ranging from $9.55 to $10.35, inclusive. The reporting person undertakes to provide Issuer, any security holder of Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in this footnote.</footnote>
        <footnote id="F2">Funds owned and managed by H Partners Management, LLC (&quot;H Management&quot;) directly own the reported securities. As a result, the Reporting Person, as Founder and Managing Member of H Management, may be deemed to have voting and dispositive power with respect to the shares of common stock held by the managed funds. Mr. Jaffer disclaims beneficial ownership of the reported securities except to the extent of his pecuniary interest therein, and this report shall not be deemed an admission that such reporting person is the beneficial owner of the securities for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose.</footnote>
        <footnote id="F3">The price reported in column 4 is a weighted average price. These shares were purchased in multiple transactions ranging from $10.69 to $11.00, inclusive. The reporting person undertakes to provide Issuer, any security holder of Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in this footnote.</footnote>
        <footnote id="F4">The price reported in column 4 is a weighted average price. These shares were purchased in multiple transactions ranging from $10.00 to $10.76, inclusive. The reporting person undertakes to provide Issuer, any security holder of Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in this footnote.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Rehan Jaffer</signatureName>
        <signatureDate>2026-11-10</signatureDate>
    </ownerSignature>
</ownershipDocument>

Congressional trading in FUN

These disclosures provide context. Their presence does not establish a link between a trader and the event.

Original documents control. Public records can be amended, and company connections may require interpretation. Nothing here is investment advice.