Filed pursuant to Rule 424(b)(3)

Registration Statement No. 333-262339

Prospectus Supplement No. 15

(To Prospectus dated March 16, 2023)

eFFECTOR Therapeutics, Inc.

This prospectus supplement updates, amends and supplements the prospectus dated March 16, 2023 (the “Prospectus”), which forms a part of our Registration Statement on Form S-1 (Registration No. 333-262339). Capitalized terms used in this prospectus supplement and not otherwise defined herein have the meanings specified in the Prospectus.

This prospectus supplement is being filed to update, amend and supplement the information included in the Prospectus with the information contained in our Current Report on Form 8-K (the “Current Report”), filed with the SEC on January 10, 2024. Accordingly, we have attached the Current Report to this prospectus supplement.

This prospectus supplement is not complete without the Prospectus. This prospectus supplement should be read in conjunction with the Prospectus, which is to be delivered with this prospectus supplement, and is qualified by reference thereto, except to the extent that the information in this prospectus supplement updates or supersedes the information contained in the Prospectus.

Our common stock and warrants are listed on the Nasdaq Capital Market under the symbols “EFTR” and “EFTRW.” On January 9, 2024, the closing price of our common stock was $0.4425 and the closing price of our warrants was $0.0681.

We are an “emerging growth company” under federal securities laws and are subject to reduced public company reporting requirements. Investing in our securities involves certain risks. See “Risk Factors” beginning on page 7 of the Prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if the Prospectus or this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is January 10, 2024.

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): January 9, 2024

eFFECTOR Therapeutics, Inc.

(Exact name of registrant as specified in its charter)

(858) 925-8215

(Registrant’s telephone number, including area code)

(Former name or former address, if changed since last report)

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:

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

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

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.  ☐

Item 3.03 Material Modification to Rights of Security Holders.

The information contained in Item 5.03 below is incorporated by reference into this Item 3.03.

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

On January 9, 2024, eFFECTOR Therapeutics, Inc. (the “Company”) filed an amendment (the “Amendment”) to its amended and restated certificate of incorporation to effectuate a reverse stock split of the Company’s common stock, par value $0.0001per share (“Common Stock”). Pursuant to the Amendment, at the effective time of 12:01 a.m. Eastern Time on January 12, 2024, each 25 shares of Common Stock issued and outstanding will be combined into one validly issued, fully paid and non-assessable share of Common Stock (the “Reverse Stock Split”). The par value per share remains the same. Trading of the Common Stock on a Reverse Stock Split-adjusted basis will begin at the opening of trading on the Nasdaq Capital Market on January 12, 2024. The new CUSIP number for the Common Stock following the Reverse Stock Split is 28202V207. The CUSIP number for the Company’s publicly traded warrants will not change.

The Reverse Stock Split will affect all stockholders uniformly and will not alter any stockholder’s percentage interest in the Company’s equity, except to the extent that the Reverse Stock Split would result in a stockholder owning a fractional share. No fractional shares will be issued in connection with the Reverse Stock Split. Stockholders who would otherwise be entitled to a fractional share of Common Stock are instead entitled to receive a proportional cash payment. In connection with the Reverse Stock Split, the total number of shares of Common Stock that the Company is authorized to issue will be proportionally reduced from 1,000,000,000 shares to 40,000,000 shares. The Reverse Stock Split will reduce the number of shares outstanding from approximately 74.9 million shares to approximately 3.0 million shares, subject to adjustment for fractional shares.

In addition, proportionate adjustments will be made to the per share exercise price and the number of shares issuable upon the exercise of all outstanding stock options and warrants to purchase shares of Common Stock and the number of shares of Common Stock reserved for issuance pursuant to the Company’s equity incentive compensation plans.

The Reverse Stock Split ratio was selected pursuant to the authority granted to the board of directors of the Company by stockholders of the Company at the Annual Meeting of Stockholders held on June 22, 2023. A copy of the Amendment filed with the Secretary of State of the State of Delaware is filed as Exhibit 3.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits

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.

Exhibit 3.1

CERTIFICATE OF AMENDMENT

OF AMENDED AND RESTATED CERTIFICATE OF INCORPORATION

OF eFFECTOR THERAPEUTICS, INC.

eFFECTOR Therapeutics, Inc. (the “Corporation”), a corporation organized and existing under and by virtue of the General Corporation Law of the State of Delaware (the “DGCL”), does hereby certify:

1. The name of the Corporation is eFFECTOR Therapeutics, Inc. The original Certificate of Incorporation of eFFECTOR Therapeutics, Inc. was filed with the Secretary of State of Delaware on October 2, 2020.

2. This Certificate of Amendment hereby amends and restates the first two paragraphs of ARTICLE IV of the Corporation’s Amended and Restated Certificate of Incorporation to read in its entirety as follows:

ARTICLE IV

CAPITAL STOCK

The Corporation is authorized to issue two classes of stock to be designated, respectively, “Common Stock” and “Preferred Stock.” The total number of shares of capital stock which the Corporation shall have authority to issue is 140,000,000. The total number of shares of Common Stock that the Corporation is authorized to issue is 40,000,000, having a par value of $0.0001 per share, and the total number of shares of Preferred Stock that the Corporation is authorized to issue is 100,000,000, having a par value of $0.0001 per share.

Effective at 12:01 a.m., Eastern Time, on January 12, 2024 (the “Effective Time”) pursuant to the DGCL of this Certificate of Amendment of the Amended and Restated Certificate of Incorporation of the Corporation, as amended, each 25 shares of the Corporation’s common stock, par value $0.0001 per share, issued and outstanding immediately prior to the Effective Time (the “Old Shares”) shall automatically be combined into one validly issued, fully paid and non-assessable share of common stock without any further action by the Corporation or the holder thereof, subject to the treatment of fractional share interests as described below (the “Reverse Stock Split”). The Corporation shall not issue fractional shares in connection with the Reverse Stock Split. Holders of Old Shares who would otherwise be entitled to receive a fraction of a share on account of the Reverse Stock Split shall receive, upon surrender of the stock certificates formerly representing the Old Shares, in lieu of such fractional share, an amount in cash equal to the product of (1) the closing sale price per share of the common stock as reported by The Nasdaq Capital Market on the last trading day preceding the Effective Time by (2) the number of Old Shares held by such holder that would otherwise have been exchanged for such fractional share interests.

3. This Certificate of Amendment was duly adopted in accordance with Section 242 of the DGCL. The Board of Directors of the Corporation duly adopted resolutions setting forth and declaring advisable this Certificate of Amendment and directed that such amendment be considered by the stockholders of the Corporation. An annual meeting of stockholders was duly called upon notice in accordance with Section 222 of the DGCL and held on June 22, 2023, at which meeting the necessary number of shares were voted in favor of such amendment. The stockholders of the Corporation duly adopted this Certificate of Amendment.

IN WITNESS WHEREOF, this Certificate of Amendment of Amended and Restated Certificate of Incorporation has been executed as of this 9th day January, 2024.