Filed pursuant to Rule 424(b)(3)
Registration Statement No. 333-262339

Prospectus Supplement No. 10

(To Prospectus dated March 21, 2022)

eFFECTOR Therapeutics, Inc.

This prospectus supplement updates, amends and supplements the prospectus dated March 21, 2022 (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 September 1, 2022. 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 August 31, 2022, the closing price of our common stock was $0.61 and the closing price of our warrants was $0.13.

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 September 1, 2022.

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): August 29, 2022

eFFECTOR Therapeutics, Inc.

(Exact name of registrant as specified in its charter)

142 North Cedros Avenue, Suite B

Solana Beach, California 92075

(Address of principal executive offices) (Zip Code)

858-422-4300

(Registrant’s telephone number, include area code)

N/A

(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 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§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. ☐

On September 1, 2022, eFFECTOR Therapeutics, Inc. (the “Company”) entered into a Controlled Equity Offering^SM Sales Agreement (the “Sales Agreement”) with Cantor Fitzgerald & Co. (the “Agent”), under which the Company may, from time to time, sell shares of the Company’s common stock having an aggregate offering price of up to $15.0 million in “at the market” offerings through the Agent. Sales of the shares of common stock, if any, will be made at prevailing market prices at the time of sale, or as otherwise agreed with the Agent. The Agent will receive a commission from the Company of 3.0% of the gross proceeds of any shares of common stock sold under the Sales Agreement.

The Company is not obligated to sell, and the Agent is not obligated to buy or sell, any shares of common stock under the Sales Agreement. No assurance can be given that the Company will sell any shares of common stock under the Sales Agreement, or, if it does, as to the price or amount of shares of common stock that it sells or the dates when such sales will take place.

In the Sales Agreement, the Company agreed to indemnify the Agent against certain liabilities, including under the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended, or to contribute to payments that the Agent may be required to make because of such liabilities. The Company and the Agent may each terminate the Sales Agreement at any time upon specified prior written notice.

The shares will be issued pursuant to the Company’s shelf registration statement on Form S-3, including the prospectus supplement contained therein, filed with the Securities and Exchange Commission (the “SEC”) on September 1, 2022 (the “Registration Statement”), once the Registration Statement is declared effective by the SEC. A copy of the Sales Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the Sales Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Sales Agreement.

A copy of the opinion of Latham & Watkins LLP relating to the validity of the shares of common stock that may be sold pursuant to the Sales Agreement is filed herewith as Exhibit 5.1.

On August 29, 2022, the Company received a letter from the Nasdaq Stock Market staff indicating that, for the last thirty consecutive business days, the bid price for the Company’s common stock had closed below the minimum $1.00 per share requirement for continued listing on The Nasdaq Capital Market under Nasdaq Listing Rule 5550(a)(2).

In accordance with Nasdaq Listing Rule 5810(c)(3)(A), the Company has been provided an initial period of 180 calendar days, or until February 27, 2023, to regain compliance. The letter states that the Nasdaq staff will provide written notification that the Company has achieved compliance with Rule 5550(a)(2) if at any time before February 27, 2023, the bid price of the Company’s common stock closes at $1.00 per share or more for a minimum of ten consecutive business days. The Nasdaq letter has no immediate effect on the listing or trading of the Company’s common stock and the common stock will continue to trade on The Nasdaq Capital Market under the symbol “EFTR.”

The Company intends to monitor the bid price of its common stock and consider available options if its common stock does not trade at a level likely to result in the Company regaining compliance with Nasdaq’s minimum bid price rule by February 27, 2023.

If the Company does not regain compliance with Rule 5550(a)(2) by February 27, 2023, the Company may be eligible for an additional 180 calendar day compliance period. To qualify, the Company would be required to meet the continued listing requirement for market value of publicly held shares and all other initial listing standards for the Nasdaq Capital Market, with the exception of the bid price requirement, and would need to provide written notice of its intention to cure the deficiency during the second compliance period, by effecting a reverse stock split, if necessary. However, if it appears to the Nasdaq staff that the Company will not be able to cure the deficiency, or if the Company is otherwise not eligible, the Nasdaq staff would notify the Company that its securities would be subject to delisting. In the event of such a notification, the Company may appeal the Nasdaq staff’s determination to delist its securities, but there can be no assurance the Nasdaq staff would grant the Company’s request for continued listing.

(d) Exhibits.

SIGNATURES

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.