Company: BLLN
Filing Date: 2025-11-06
Form Type: S-8
Source: 0001193125-25-268580
Chunk: 0

Company: BillionToOne, Inc.
Filing Date: 2025-11-06
Form: S-8
Chunk 0
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As filed with the Securities and Exchange Commission on November 6, 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 BILLIONTOONE, INC. (Exact name of Registrant as specified in its charter)

| Delaware                        |     | 81-1082020         |
| (State or other jurisdiction of 
 incorporation or organization)  |     | -IRS Employer      
 Identification No. |

1035 O’Brien Drive Menlo Park, CA 94025 (650) 460-2551 (Address of Principal Executive Offices) BillionToOne, Inc. 2018 Stock Plan BillionToOne, Inc. 2025 Equity Incentive Plan BillionToOne, Inc. 2025 Employee Stock Purchase Plan (Full title of Plan) Oguzhan Atay Chief Executive Officer BillionToOne, Inc. 1035 O’Brien Drive Menlo Park, CA 94025 (Name and address of agent for service) (650) 460-2552 (Telephone number, including area code, of agent for service) Copies to:

| Nicholas B. Harley                  
 Andrew D. Thorpe                    
 Alexa Belonick                      
 Elena M. Vespoli                    
 Gunderson Dettmer Stough Villeneuve 
 Franklin & Hachigian, LLP           
 1 Bush Street #1200 San             
 Francisco, CA 94104 (415) 978-9803  |     | Thomas Lynch         
 General Counsel      
 BillionToOne, Inc.   
 1035 O’Brien Drive   
 Menlo Park, CA 94025 
 (650) 460-2552       |

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 definitions 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