<?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>
[{"type": "144", "sequence": "1", "filename": "primary_doc.xml", "secsgml_size_bytes": 7194}]
144
0001863676
XXXXXXXX
LIVE
0001845337
DAY ONE BIOPHARMACEUTICALS INC
001-40431
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
6504840899
SAMUEL BLACKMAN
Officer
COMMON
Morgan Stanley Smith Barney LLC Executive Financial Services
1 New York Plaza
8th Floor
New York
NY
10004
50000
819500.00
100846294
11/11/2024
NASDAQ
Common
05/31/2021
RSA
Issuer
N
50000
05/31/2021
N/A
N
10b5-1 Sales Plan for SAMUEL BLACKMAN
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
COMMON
11/08/2024
18255
295229.15
10b5-1 Sales Plan for SAMUEL BLACKMAN
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
COMMON
11/07/2024
11245
180286.46
10b5-1 Sales Plan for SAMUEL BLACKMAN
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
COMMON
10/31/2024
500
8010.00
10b5-1 Sales Plan for SAMUEL BLACKMAN
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
COMMON
09/10/2024
30000
426675.00
SAMUEL BLACKMAN
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
Common
08/16/2024
2232
31245.99
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.
11/11/2024
10/10/2023
/s/ Samuel Blackman