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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-11-14</periodOfReport>

    <issuer>
        <issuerCik>0000041719</issuerCik>
        <issuerName>Magnera Corp</issuerName>
        <issuerTradingSymbol>MAGN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001580949</rptOwnerCik>
            <rptOwnerName>Curless Michael S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>9335 HARRIS CORNERS PKWY</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 300</rptOwnerStreet2>
            <rptOwnerCity>CHARLOTTE</rptOwnerCity>
            <rptOwnerState>NC</rptOwnerState>
            <rptOwnerZipCode>28269</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2025-11-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5871</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2026-11-14</value>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-11-14</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock, Par Value $.01</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5871</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5871</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Not applicable to this transaction.  RSUs have no value until all restrictions lapse on the final vesting date.</footnote>
        <footnote id="F2">To align the director equity schedule with the Annual Shareholders Meeting moving forward, this grant represents a prorated value of the Annual Director Award for the period between the 2024 RSU lapse reported on November 4, 2025 and the Annual Shareholder Meeting in 2026, at which time directors will be granted a full 2026 Annual Director Award upon their reelection.</footnote>
        <footnote id="F3">This 2025 Prorated Director Award vests in full one year from the grant date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Laura A. Jones, attorney-in-fact for Michael Curless</signatureName>
        <signatureDate>2025-11-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
