As filed with the Securities and Exchange Commission on February 13, 2023

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

DAY ONE BIOPHARMACEUTICALS, INC.

(Exact name of Registrant as specified in its charter)

2000 Sierra Point Parkway, Suite 501

Brisbane, CA 94005

(650) 484-0899

(Address of Principal Executive Offices) (Zip Code)

2021 Equity Incentive Plan

2021 Employee Stock Purchase Plan

(Full title of the plans)

Jeremy Bender, Ph.D., M.B.A.

Chief Executive Officer

Day One Biopharmaceuticals, Inc.

2000 Sierra Point Parkway, Suite 501

Brisbane, CA 94005

(Name and address of agent for service)

(650) 484-0899

(Telephone number, including area code, of agent for service)

Please send copies of all communications to:

Effie Toshav, Esq.

Robert Freedman, Esq.

Julia Forbess, Esq.

Fenwick & West LLP

555 California Street

San Francisco, California 94104

(415) 875-2300

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definition of “large accelerated filer”, “accelerated filer”, “smaller reporting company”, and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

REGISTRATION OF ADDITIONAL SHARES

PURSUANT TO GENERAL INSTRUCTION E

Day One Biopharmaceuticals, Inc. (the “Registrant”) is filing this Registration Statement with the Securities and Exchange Commission (the “Commission”) to register (a) 3,672,908 additional shares of common stock granted or available for issuance under the Registrant’s 2021 Equity Incentive Plan (the “2021 EIP”), pursuant to the provision of the 2021 EIP providing for an annual 5% automatic increase in the number of shares reserved for issuance, and (b) 734,581 additional shares of common stock available for issuance under the Registrant’s 2021 Employee Stock Purchase Plan (the “2021 ESPP”), pursuant to the provision of the 2021 ESPP providing for an annual 1% automatic increase in the number of shares reserved for issuance.

In accordance with General Instruction E of Form S-8, and only with respect to the common stock issuable under the 2021 EIP and 2021 ESPP, this Registration Statement hereby incorporates by reference the contents of the Registrant’s Registration Statements on Form S-8 filed with the Commission on May 26, 2021 (Registration No. 333-256521) and March 7, 2022 (Registration No. 333-263343) to the extent not superseded hereby.

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PART II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.

The following documents filed by Day One Biopharmaceuticals, Inc. (the “Registrant”) with the Commission pursuant to the Securities Act and the Securities Exchange Act of 1934, as amended (the “Exchange Act”) are incorporated herein by reference:

All documents filed by the Registrant pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the filing of this Registration Statement and prior to the filing of a post-effective amendment which indicates that all securities offered hereby have been sold or which deregisters all securities then remaining unsold, shall be deemed to be incorporated by reference into this Registration Statement and to be a part hereof from the date of filing such documents, except as to documents or information deemed to have been furnished and not filed in accordance with the rules of the Commission. Unless expressly incorporated into this Registration Statement, a report deemed to be furnished but not filed on Form 8-K prior or subsequent to the date hereof shall not be incorporated by reference into this Registration Statement. Any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes of this Registration Statement to the extent that a statement contained in any subsequently filed document which also is deemed to be incorporated by reference herein modifies or supersedes such statement.

Item 8. Exhibits.

The following exhibits are filed herewith:

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on this 13th day of February, 2023.

POWER OF ATTORNEY

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Jeremy Bender and Charles N. York II, and each of them, as his or her true and lawful attorney-in-fact and agent with full power of substitution, for him or her in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement on Form S-8, and to file the same, with all exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully for all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorney-in-fact and agent, or his or her substitute, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following persons on behalf of the Registrant in the capacities and on the date indicated.

Exhibit 5.1

February 13, 2023

Day One Biopharmaceuticals, Inc.

2000 Sierra Point Parkway, Suite 501

Brisbane, CA 94005

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

As counsel to Day One Biopharmaceuticals, Inc., a Delaware corporation (the “Company”), we have examined the Registration Statement on Form S-8 (the “Registration Statement”) to be filed by the Company with the Securities and Exchange Commission (the “Commission”) on or about February 13, 2023 in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of an aggregate of 4,407,489 shares (the “Shares”) of the Company’s Common Stock, $0.0001 par value per share (the “Common Stock”), subject to issuance by the Company (a) upon the exercise or settlement of equity awards to be granted under the Company’s 2021 Equity Incentive Plan (the “2021 EIP”) and (b) pursuant to purchase rights to acquire shares of Common Stock to be granted under the Company’s 2021 Employee Stock Purchase Plan (the “ESPP”, and together with the 2021 EIP, the “Plans”).

As to matters of fact relevant to the opinions rendered herein, we have examined such documents, certificates and other instruments which we have deemed necessary or advisable, including a certificate addressed to us and dated the date hereof executed by the Company (the “Opinion Certificate”). We have not undertaken any independent investigation to verify the accuracy of any such information, representations or warranties or to determine the existence or absence of any fact, and no inference as to our knowledge of the existence or absence of any fact should be drawn from our representation of the Company or the rendering of the opinion set forth below. We have not considered parol evidence in connection with any of the agreements or instruments reviewed by us in connection with this letter.

In our examination of documents for purposes of this letter, we have assumed, and express no opinion as to, the genuineness and authenticity of all signatures on original documents, the authenticity and completeness of all documents submitted to us as originals, that each document is what it purports to be, the conformity to originals of all documents submitted to us as copies or facsimile copies, the absence of any termination, modification or waiver of or amendment to any document reviewed by us (other than as has been disclosed to us), the legal competence or capacity of all persons or entities (other than the Company) executing the same and (other than the Company) the due authorization, execution and delivery of all documents by each party thereto. We have also assumed the conformity of the documents filed with the Commission via the Electronic Data Gathering, Analysis and Retrieval System (“EDGAR”), except for required EDGAR formatting changes, to physical copies submitted for our examination.

The opinions in this letter are limited to the existing General Corporation Law of the State of Delaware now in effect. We express no opinion with respect to any other laws.

Based upon, and subject to, the foregoing, it is our opinion that the Shares, when issued and sold by the Company in accordance with the terms (including, without limitation, payment and authorization provisions) of the applicable Plan and the applicable form of award agreement thereunder, against the Company’s receipt of payment therefor (in an amount and type of consideration not less than the par value per Share), and duly registered on the books of the transfer agent and registrar for the Shares in the name or on behalf of the holders thereof, will be validly issued, fully paid and non-assessable.

We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to all references to us, if any, in the Registration Statement, the prospectuses constituting a part thereof and any amendments thereto. We do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

This opinion is intended solely for your use in connection with the issuance and sale of the Shares subject to the Registration Statement and is not to be relied upon for any other purpose. In providing this letter, we are opining only as to the specific legal issues expressly set forth above, and no opinion shall be inferred as to any other matter or matters. This opinion is rendered on, and speaks only as of, the date of this letter first written above, and does not address any potential change in facts or law that may occur after the date of this opinion letter. We assume no obligation to advise you of any fact, circumstance, event or change in the law or the facts that may hereafter be brought to our attention, whether or not such occurrence would affect or modify any of the opinions expressed herein.

Exhibit 23.2

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Registration Statement (Form S-8) pertaining to the 2021 Equity Incentive Plan and 2021 Employee Stock Purchase Plan of Day One Biopharmaceuticals, Inc. of our report dated March 7, 2022 with respect to the consolidated financial statements of Day One Biopharmaceuticals, Inc. included in its Annual Report (Form 10-K) for the year ended December 31, 2021, filed with the Securities and Exchange Commission.

/s/ Ernst & Young LLP

San Mateo, California

February 13, 2023

Exhibit 107

Calculation of Filing Fee Tables

Form S-8

(Form Type)

Day One Biopharmaceuticals, Inc.

(Exact Name of Registrant as Specified in its Charter)

Newly Registered Securities