<?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/ownership" xmlns:ns2="http://www.sec.gov/edgar/common">
    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001863676</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0001845337</issuerCik>
            <issuerName>DAY ONE BIOPHARMACEUTICALS INC</issuerName>
            <secFileNumber>001-40431</secFileNumber>
            <issuerAddress>
                <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                <ns2:city>Brisbane </ns2:city>
                <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                <ns2:zipCode>94005</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>6504840899</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>SAMUEL BLACKMAN</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>8th Floor</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>50000</noOfUnitsSold>
            <aggregateMarketValue>819500.00</aggregateMarketValue>
            <noOfUnitsOutstanding>100846294</noOfUnitsOutstanding>
            <approxSaleDate>11/11/2024</approxSaleDate>
            <securitiesExchangeName>NASDAQ</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>05/31/2021</acquiredDate>
            <natureOfAcquisitionTransaction>RSA</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>50000</amountOfSecuritiesAcquired>
            <paymentDate>05/31/2021</paymentDate>
            <natureOfPayment>N/A</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>10b5-1 Sales Plan for SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <saleDate>11/08/2024</saleDate>
            <amountOfSecuritiesSold>18255</amountOfSecuritiesSold>
            <grossProceeds>295229.15</grossProceeds>
        </securitiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>10b5-1 Sales Plan for SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <saleDate>11/07/2024</saleDate>
            <amountOfSecuritiesSold>11245</amountOfSecuritiesSold>
            <grossProceeds>180286.46</grossProceeds>
        </securitiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>10b5-1 Sales Plan for SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <saleDate>10/31/2024</saleDate>
            <amountOfSecuritiesSold>500</amountOfSecuritiesSold>
            <grossProceeds>8010.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>10b5-1 Sales Plan for SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <saleDate>09/10/2024</saleDate>
            <amountOfSecuritiesSold>30000</amountOfSecuritiesSold>
            <grossProceeds>426675.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>08/16/2024</saleDate>
            <amountOfSecuritiesSold>2232</amountOfSecuritiesSold>
            <grossProceeds>31245.99</grossProceeds>
        </securitiesSoldInPast3Months>
        <remarks>Sales that were executed on November 11, 2024 are being reported on this Form 144 filed on November 12, 2024 given the EDGAR system was not accepting filings November 11, 2024 in observance of the federal holiday.</remarks>
        <noticeSignature>
            <noticeDate>11/11/2024</noticeDate>
            <planAdoptionDates>
                <planAdoptionDate>10/10/2023</planAdoptionDate>
            </planAdoptionDates>
            <signature>/s/ Samuel Blackman</signature>
        </noticeSignature>
    </formData>
</edgarSubmission>
