BIOLARGO, INC.

As filed with the Securities and Exchange Commission on April 9, 2026

Registration No. 333-

UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORMS-1

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

BIOLARGO, INC.

(Exact name of registrant as specified in its charter)

BioLargo, Inc.

14921 Chestnut St.

Westminster, CA 92683

(888)400-2863

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Copy to:

Gilbert Bradshaw, Esq.

Corporate Securities Legal LLP

650 Town Center Drive, Suite 680

Costa Mesa, CA 92626

Tel: (949) 752-1100/Fax: (949) 752-1144

gbradshaw@securitieslegal.com

Agents and Corporations, Inc.

1201 Orange Street, Suite 600

Wilmington, DE 19801

(302) 575-0877

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale to the public:

From time to time after this registration statement is declared effective.

If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box: ☒

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company or an emerging growth company. See the definitions 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 to Section 7(a)(2)(B) of the Securities Act. ☐

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. These securities may not be sold until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

PROSPECTUS (Subject to Completion)

Dated: [--], 2026

PROSPECTUS

31,730,651 shares of common stock

This prospectus relates to the offer and sale of up to 31,730,651 shares of common stock, par value $0.00067, of BioLargo, Inc., a Delaware corporation, by the selling stockholders identified herein (referred to collectively herein as the “selling stockholders,” or individually as a “selling stockholder”). These shares are issuable upon exercise of warrants that were issued to the selling stockholders between April 16, 2021, and January 6, 2026, at an average exercise price of $0.265 per share.

We are not selling any securities under this prospectus and will not receive any of the proceeds from the sale of shares by the selling stockholders. We may receive up to $8,416,462 aggregate gross proceeds in the event the warrants are exercised in full.

After exercise of the warrants, the selling stockholders may sell the shares of common stock described in this prospectus in a number of different ways and at varying prices. See “Plan of Distribution” for more information about how the selling stockholders may sell the shares of common stock being registered pursuant to this prospectus. Each selling stockholder may be considered an “underwriter” within the meaning of Section 2(a)(11) of the Securities Act of 1933, as amended.

We will pay the expenses incurred in registering the shares, including legal and accounting fees. See “Plan of Distribution”.

Since January 23, 2008, our common stock has been quoted on the OTC Markets Group Inc. (formerly OTC Bulletin Board) under the trading symbol “BLGO.” On April 8, 2026, the last reported sale price of our common stock on the OTC Markets Group Inc.’s OTCQX® Best Market was $0.15.

The securities offered in this prospectus involve a high degree of risk. You should consider the risk factors beginning on page 5 before purchasing our common stock.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is [--], 2026