As filed with the Securities and Exchange Commission on February 6, 2025.

Registration No. 333–   

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Sionna Therapeutics, Inc.

(Exact name of registrant as specified in its charter)

21 Hickory Drive, Suite 500

Waltham, MA 02451

(617) 819-2020

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

Michael Cloonan

President and Chief Executive Officer

Sionna Therapeutics, Inc.

21 Hickory Drive, Suite 500

Waltham, MA 02451

(617) 819-2020

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

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this registration statement.

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. ☒ 333-284352

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, 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 pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

The Registration Statement shall become effective upon filing in accordance with Rule 462(b) promulgated under the Securities Act of 1933, as amended.

EXPLANATORY NOTE AND INCORPORATION BY REFERENCE

This Registration Statement is being filed pursuant to Rule 462(b) under the Securities Act of 1933, as amended (the “Securities Act”), for the sole purpose of increasing the aggregate number of shares of common stock offered by Sionna Therapeutics, Inc. (the “Registrant”) by 2,029,409 shares, 264,705 of which are subject to purchase upon exercise of the underwriters’ option to purchase additional shares of the Registrant’s common stock. The contents of the Registration Statement on Form S-1, as amended (File No. 333-284352), filed by the Registrant with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act, which was declared effective by the Commission on February 6, 2025, including all exhibits thereto, are incorporated by reference into this Registration Statement.

The required opinion and consents are listed on an Exhibit Index attached hereto and filed herewith.

EXHIBIT INDEX

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this registration statement on Form S-1 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Waltham, Massachusetts, on the 6th day of February, 2025.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-1 has been signed by the following person in the capacities and on the date indicated.

Exhibit 5.1

February 6, 2025

Sionna Therapeutics, Inc.

21 Hickory Drive, Suite 500

Waltham, MA 02451

We have acted as counsel to you in connection with your filing of (i) a Registration Statement on Form S-1 (File No. 333-284352) (as amended or supplemented, the “Initial Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), and (ii) a second Registration Statement on Form S-1 filed pursuant to Rule 462(b) promulgated under the Securities Act (the “462(b) Registration Statement,” and together with the Initial Registration Statement, the “Registration Statement”). This opinion letter is being furnished to you in connection with your filing of the 462(b) Registration Statement relating to the registration of the offering by Sionna Therapeutics, Inc., a Delaware corporation (the “Company”), of up to 2,029,409 shares (the “Shares”) of the Company’s Common Stock, $0.001 par value per share, including Shares purchasable by the underwriters upon their exercise of an over-allotment option granted to the underwriters by the Company. The Shares are being sold to the several underwriters named in, and pursuant to, an underwriting agreement among the Company and such underwriters (the “Underwriting Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinions set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when delivered and paid for in accordance with the terms of the Underwriting Agreement, will be validly issued, fully paid and non-assessable.

This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the 462(b) Registration Statement and to the references to our firm under the caption “Legal Matters” in the Initial Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this registration statement on Form S-1 of our report dated September 12, 2024 (February 3, 2025, as to the effects of the reverse stock split discussed in Note 16), relating to the financial statements of Sionna Therapeutics, Inc. appearing in registration statement No. 333-284352 on Form S-1 of Sionna Therapeutics, Inc. We also consent to the reference to us under the heading “Experts” in registration statement No. 333-284352 on Form S-1.

/s/ Deloitte & Touche LLP

Boston, Massachusetts

February 6, 2025

Exhibit 107

Calculation of Filing Fee Table

Form S-1

(Form Type)

Sionna Therapeutics, Inc.

(Exact Name of Registrant as Specified in its Charter)

Table 1: Newly Registered Securities