UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): July 9, 2018

SPERO THERAPEUTICS, INC.

(Exact Name of registrant as specified in its charter)

675 Massachusetts Avenue, 14th Floor

Cambridge, Massachusetts 02139

(Address of principal executive offices and zip code)

Registrant’s telephone number, including area code: (857) 242-1600

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (17 CFR §230.405) or Rule 12b-2 of the Securities Exchange Act of 1934 (17 CFR §240.12b-2).

Emerging growth company  ☒

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 13(a) of the Exchange Act.  ☒

On July 9, 2018, Spero Therapeutics, Inc. (the “Company”) issued a press release announcing interim data from its Phase 1 dose-selection clinical trial of SPR994. A copy of the press release is attached hereto as Exhibit 99.1. Additional information about these interim results is set forth in Exhibit 99.2.

(d) Exhibits.

Forward Looking Statements

This Current Report on Form 8-K may contain forward-looking statements. These statements include, but are not limited to, statements about the initiation, timing, progress and results of the Company’s preclinical studies and clinical trials and its research and development programs, including statements regarding management’s assessment of the results of such preclinical studies and clinical trials (including the ongoing SPR994 Phase 1 clinical trial), the timing of clinical data, the Company’s cash forecast and anticipated expenses, the sufficiency of its cash resources and the availability of additional non-dilutive funding from government agencies beyond any initially funded awards. In some cases, forward-looking statements can be identified by terms such as “may,” “will,” “should,” “expect,” “plan,” “aim,” “anticipate,” “could,” “intent,” “target,” “project,” “contemplate,” “believe,” “estimate,” “predict,” “potential” or “continue” or the negative of these terms or other similar expressions. Actual results may differ materially from those indicated by such forward-looking statements as a result of various important factors, including whether results obtained in preclinical studies and clinical trials will be indicative of results obtained in future clinical trials; the final results of the SPR994 Phase 1 clinical trial; whether the Company’s product candidates will advance through the preclinical development and clinical trial process on a timely basis, or at all; whether the results of such trials will warrant submission for approval from the U.S. Food and Drug Administration or equivalent foreign regulatory agencies; whether the Company’s cash resources will be sufficient to fund its continuing operations for the periods and/or trials anticipated; whether the conditions to our receipt of conditional funding under our government contracts are satisfied or such conditional funding is awarded to us and other factors discussed in the “Risk Factors” set forth in filings that the Company periodically makes with the U.S. Securities Exchange Commission. The forward-looking statements included in this Current Report on Form 8-K represent the Company’s views as of the date of this Current Report on Form 8-K. The Company anticipates that subsequent events and developments will cause its views to change. However, while the Company may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so. These forward-looking statements should not be relied upon as representing the Company’s views as of any date subsequent to the date of this Current Report on Form 8-K.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.