BillionToOne, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.)

Filed by the Registrant☒

Filed by a party other than the Registrant ☐

Check the appropriate box:

BillionToOne, Inc.

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

1035 O’Brien Drive

Menlo Park, CA 94025

April 21, 2026

Dear BillionToOne Stockholder:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders of BillionToOne, Inc. (the Company) that will be held on Wednesday, June 10, 2026 at 9:00 a.m. PDT (the Annual Meeting). The Annual Meeting will be held virtually via live audio webcast on the internet at www.virtualshareholdermeeting.com/BLLN2026. Even though our Annual Meeting will be held virtually, stockholders will still have the ability to participate in the meeting and vote their shares at the Annual Meeting if they wish.

Details regarding the Annual Meeting and the business to be conducted are described in the accompanying proxy materials. We encourage you to read this information carefully, along with our annual report on Form 10-K for the year ended December 31, 2025. Your vote is important. Whether or not you plan to attend the Annual Meeting, we hope you will vote as soon as possible. You may vote over the internet, by telephone or by mailing a proxy card, if you have requested one. Please review the instructions on the Notice of Internet Availability of Proxy Materials you received in the mail regarding each of these voting options.

Thank you for your ongoing support of the Company.

Very truly yours,

Oguzhan Atay, PhD

Chair of the Board of Directors, Chief Executive Officer and Co-Founder

NOTICE OF 2026 ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ON JUNE 10, 2026

ITEMS OF BUSINESS

To transact such other business as may properly come before the Annual Meeting or any adjournment thereof.

These items of business are more fully described in the proxy statement accompanying this notice.

ADJOURNMENTS AND POSTPONEMENTS

Any action on the items of business described above may be considered at the Annual Meeting at the time and on the date specified above or at any time and date to which the Annual Meeting may be properly adjourned or postponed.

VOTING

Your vote is very important. Whether or not you plan to attend the Annual Meeting, we encourage you to read the proxy statement and vote on the internet or by telephone or submit your proxy card, if you have requested one, as soon as possible. For specific instructions on how to vote your shares, please refer to the section entitled “Questions and Answers about Procedural Matters.”

A Notice of Internet Availability of Proxy Materials (Notice) has been mailed to stockholders of record on or about April 21, 2026. The Notice contains instructions on how to access our proxy statement for our 2026 Annual Meeting of Stockholders and our annual report on Form 10-K for the year ended December 31, 2025 (together, the proxy materials). The Notice also provides instructions on how to vote online, by telephone or by mail and includes instructions on how to receive a paper copy of the proxy materials by mail. The proxy materials can be accessed directly at the following internet address: www.proxyvote.com.

By order of the Board of Directors of BillionToOne, Inc.,

Oguzhan Atay, PhD

Chair of the Board of Directors, Chief Executive Officer and Co-Founder

This notice of annual meeting, proxy statement and accompanying form of proxy card are being made available on or about April 21, 2026.

Table of Contents

PROXY STATEMENT

For 2026 Annual Meeting of Stockholders

This proxy statement is furnished in connection with the solicitation of proxies by our Board of Directors for use at the 2026 Annual Meeting of Stockholders (the Annual Meeting) to be held at 9:00 am PDT on Wednesday, June 10, 2026, and any postponements or adjournments thereof. The Annual Meeting will be held virtually via a live audio webcast on the internet at www.virtualshareholdermeeting.com/BLLN2026. There will not be a physical meeting location available for in-person participation. We believe holding our Annual Meeting online will facilitate greater stockholder attendance while still providing comparable rights and opportunities to participate, including the ability to ask questions, as a stockholder would have if he, she or they were attending our Annual Meeting in person. Beginning on or about April 21, 2026, we mailed to our stockholders a Notice of Internet Availability of Proxy Materials (Notice) containing instructions on how to access our proxy materials. As used in this proxy statement, the terms “Company,” “we,” “us,” and “our” mean BillionToOne, Inc. unless the context indicates otherwise. Unless otherwise noted or unless the context provides otherwise, all references in this proxy statement to our “common stock” refer to our voting Class A common stock and Class B common stock.