Company: DAWN
Filing Date: 2025-01-10
Form Type: S-8
Source: 0001193125-25-004434
Chunk: 0

Company: Day One Biopharmaceuticals, Inc.
Filing Date: 2025-01-10
Form: S-8
Chunk 0
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As filed with the Securities and Exchange Commission on January 10, 2025 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)

| Delaware                        |     | 83-2415215             |
| (State or other jurisdiction of |     | (I.R.S. Employer       |
| incorporation or organization)  |     | Identification Number) |

1800 Sierra Point Parkway, Suite 200 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. 1800 Sierra Point Parkway, Suite 200 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-acceleratedfiler, 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-2of the Exchange Act.

| Large accelerated filer |     | ☒ |     | Accelerated filer         |     | ☐ |
| Non-accelerated filer   |     | ☐ |     | Smaller reporting company |     | ☐ |
|                         |     |   |     | Emerging growth company   |     | ☐ |

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.