UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-A

FOR REGISTRATION OF CERTAIN CLASSES OF SECURITIES

PURSUANT TO SECTION 12(b) OR 12(g) OF THE

SECURITIES EXCHANGE ACT OF 1934

Securities to be registered pursuant to Section  12(b) of the Act :

If this form relates to the registration of a class of securities pursuant to Section 12(b) of the Exchange Act and is effective pursuant to General Instruction A.(c) or (e), check the following box.  ☒

If this form relates to the registration of a class of securities pursuant to Section 12(g) of the Exchange Act and is effective pursuant to General Instruction A.(d) or (e), check the following box.  ☐

If this form relates to the registration of a class of securities concurrently with a Regulation A offering, check the following box. ☐

Securities Act registration statement or Regulation A offering statement file number to which this form relates: 333-220858

Securities to be registered pursuant to Section 12(g) of the Act: None.

Item 1. Description of Registrant’s Securities to be Registered.

Spero Therapeutics, Inc. (the “ Registrant ”) hereby incorporates by reference the description relating to the Registrant’s common stock, $0.001 par value per share, to be registered hereunder set forth in the section titled “Description of Capital Stock” in the prospectus included in the Registrant’s Registration Statement on Form S-1 (File No. 333-220858), as originally filed with the U.S. Securities and Exchange Commission (the “ Commission ”) on October 6, 2017, as subsequently amended (the “ Registration Statement ”). Any form of prospectus subsequently filed by the Registrant with the Commission pursuant to Rule 424(b) under the Securities Act of 1933, as amended, that constitutes part of the Registration Statement shall be deemed to be incorporated herein by reference.

Item 2. Exhibits.

Pursuant to the Instructions as to Exhibits for Form 8-A, no exhibits are required to be filed because no other securities of the Registrant are registered on The Nasdaq Stock Market LLC and the securities registered hereby are not being registered pursuant to Section 12(g) of the Securities Exchange Act of 1934, as amended.

SIGNATURE

Pursuant to the requirements of Section 12 of the Securities Exchange Act of 1934, the Registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereto duly authorized.

Dated: October 30, 2017