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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-06-28</periodOfReport>

    <issuer>
        <issuerCik>0000097745</issuerCik>
        <issuerName>THERMO FISHER SCIENTIFIC INC.</issuerName>
        <issuerTradingSymbol>TMO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001214212</rptOwnerCik>
            <rptOwnerName>SPERLING SCOTT M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THOMAS H. LEE PARTNERS, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>100 FEDERAL STREET, 35TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02110</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2025-06-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>102.35</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>102.35</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>17337.12</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Convertible into Common Stock on a 1-for-1 basis.</footnote>
        <footnote id="F2">Represents stock units credited to the Reporting Person's account as of June 28, 2025, at a price of $408.28 per unit pursuant to the Issuer's Deferred Compensation Plan for Directors (the &quot;Plan&quot;). Directors' retainers are deferred quarterly under the Plan as Common Stock units based on the closing price of the stock as of the quarter end.  The shares are distributable as stock upon cessation of director service (for any reason) or a change of control.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Melodie T. Morin, Attorney-in-Fact for Scott M. Sperling</signatureName>
        <signatureDate>2025-07-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0508
4
2025-06-28
0000097745
THERMO FISHER SCIENTIFIC INC.
TMO
0001214212
SPERLING SCOTT M
C/O THOMAS H. LEE PARTNERS, L.P.
100 FEDERAL STREET, 35TH FLOOR
BOSTON
MA
02110
1
0
Phantom Stock Units
2025-06-28
4
A
0
102.35
A
Common Stock
102.35
17337.12
D
Convertible into Common Stock on a 1-for-1 basis.
Represents stock units credited to the Reporting Person's account as of June 28, 2025, at a price of $408.28 per unit pursuant to the Issuer's Deferred Compensation Plan for Directors (the "Plan"). Directors' retainers are deferred quarterly under the Plan as Common Stock units based on the closing price of the stock as of the quarter end.  The shares are distributable as stock upon cessation of director service (for any reason) or a change of control.
/s/ Melodie T. Morin, Attorney-in-Fact for Scott M. Sperling
2025-07-01