DEEP FISSION, INC.

As filed with the U.S. Securities and Exchange Commission on May 20, 2026.

Registration Number 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM****S-1

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933

(Exact name of registrant as specified in its charter)

2001 Addison Street, Suite 300

Berkeley, California 94704

(707) 400-0778

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

Elizabeth Muller

President and Chief Executive Officer

2001 Addison Street,

Suite 300

Berkeley, California 94704

(707) 400-0778

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

Copies to:

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

(Approximate date of commencement of proposed sale to the public)

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, please 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, 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 pursuant to Section 7(a)(2)(B) of the Exchange 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, as amended, or until this 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 preliminary prospectus is not complete and may be changed. The securities described in this prospectus may not be sold until the Registration Statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell these securities and is not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED MAY 20, 2026

PRELIMINARY PROSPECTUS

6,000,000 Shares

Deep Fission, Inc. (“Deep Fission”, the “Company”, “we”, “our”, or “us”) is offering 6,000,000 shares of our common stock, par value $0.0001 per share (our “common stock”).

There is not currently, and there has never been, any established public trading market for our common stock and this will be our first listed public offering. We have applied to list our common stock on the Nasdaq Global Market (“Nasdaq”) under the symbol “FISN”, and we will not consummate this offering of our common stock unless our common stock is approved for listing on Nasdaq.

We anticipate a public offering price between $24.00 and $26.00 per share of our common stock offered hereby. The public offering price of the shares of our common stock offered hereby will be by agreement between us and the representatives of the underwriters (the “Representatives”), and may not be indicative of the price at which shares of our common stock will trade in the public market after this offering.

We are an “emerging growth company” and a “smaller reporting company” as defined under the federal securities laws and, as such, have elected to comply with reduced public company reporting requirements. See “Prospectus Summary - Implications of Being an Emerging Growth Company and a Smaller Reporting Company.”

Investing in our common stock involves a high degree of risk. See “Risk Factors” beginning on page 32 to read about factors you should consider before buying shares of our common stock.

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

(1)See the section titled “Underwriting” for additional information regarding underwriting compensation.

This offering is being conducted on a firm commitment basis. The underwriters are obligated to take and pay for all the shares of common stock offered by this prospectus if any such shares are taken.

We have granted the underwriters the option, solely to cover over-allotments (if any) in this offering, to purchase up to 900,000 additional shares of our common stock, at the public offering price set forth above less the underwriting discounts set forth above. The underwriters can exercise this right at any time and from time to time, in whole or in part, within 30 days after the date of this prospectus.

At our request, the underwriters have reserved up to 300,000 shares of our common stock, or 5% of the shares offered hereby, for sale at the public offering price, through a directed share program to our directors, officers, certain of our employees and other individuals we identify. For more information on our directed share program, see “Underwriting—Directed Share Program.”

Delivery of the shares of common stock will be made on or about , 2026 through the book-entry facilities of The Depository Trust Company.

The date of this prospectus is , 2026