As filed with the Securities and Exchange Commission on July 11, 2018

Registration No. 333-                

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

SELLAS Life Sciences Group, Inc.

(Exact Name of Registrant as Specified in Its Charter)

315 Madison Avenue, 4th Floor

New York, NY 10017

(917) 438-4353

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Angelos M. Stergiou, M.D., Sc.D., h.c.

President and Chief Executive Officer

SELLAS Life Sciences Group, Inc.

315 Madison Avenue, 4th Floor

New York, NY 10017

(917) 438-4353

(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, as amended (the “Securities Act”), 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. ☒ (File No. 333-225140)

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. ☐

CALCULATION OF REGISTRATION FEE

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

EXPLANATORY NOTE AND

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

This Registration Statement is being filed with the Securities and Exchange Commission (the “Commission”) with respect to the registration of (i) additional shares of common stock, par value $0.0001 per share, of SELLAS Life Sciences Group, Inc., a Delaware corporation (“Common Stock”), (ii) pre-funded warrants to purchase shares of Common Stock, including shares of Common Stock underlying such pre-funded warrants and (iii) common warrants to purchase shares of Common Stock, including shares of Common Stock underlying such common warrants, pursuant to Rule 462(b) under the Securities Act of 1933, as amended. This Registration Statement incorporates by reference the contents of, including all amendments and exhibits thereto and all information incorporated by reference therein, the Registration Statement on Form S-1 (Registration No. 333-225140), which was declared effective by the Commission on July 11, 2018, and is being filed solely for the purpose of increasing the aggregate offering price of securities to be offered in the public offering by $8,078,000.

EXHIBIT INDEX

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the Registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, New York, on the 11^th day of July, 2018.

Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

*Pursuant to Power of Attorney