Filed pursuant to Rule 424(b)(3)

Registration No. 333-256938

PROSPECTUS SUPPLEMENT NO. 4

(to Prospectus dated June 16, 2021)

Hydrofarm Holdings Group, Inc.

3,369,138 Shares of Common stock

This prospectus supplement supplements the prospectus dated June 16, 2021 (the “Prospectus”), which forms a part of our registration statement on Form S-1 (No. 333-256938). This prospectus supplement is being filed to update and supplement the information in the Prospectus with the information contained in our Current Report on Form 8-K, filed with the Securities and Exchange Commission on July 23, 2021 (the “Current Report”). Accordingly, we have attached the Current Report to this prospectus supplement.

The Prospectus and this prospectus supplement relates to the offer and sale from time to time of up to 3,369,138 shares of our common stock by the selling stockholders listed on page 26 of the Prospectus. The number of shares offered for sale by the selling stockholders consists of up to 3,369,138 shares of our common stock currently issuable upon the exercise of warrants held by the selling stockholders, which were issued in connection with a private placement of units, each consisting of a share of common stock and a warrant to purchase an additional one-half (1/2) share of common stock, which concluded on October 30, 2018. The shares of our common stock offered hereby are issuable upon the exercise of warrants issued by us in a series of private placement transactions completed prior to the filing of the registration statement containing the Prospectus.

Our common stock is traded on The Nasdaq Global Select Market under the symbol “HYFM.” On July 22, 2021, the closing price of our common stock was $48.54 per share.

We are an “emerging growth company” under applicable federal securities laws and will be subject to reduced public company reporting requirements.

Investing in our common stock is highly speculative and involves a high degree of risk. See “Risk Factors” beginning on page 21 of the Prospectus to read about factors you should consider before buying shares of our common stock.

Neither the Securities and Exchange Commission nor any other regulatory body has approved or disapproved of these securities or passed upon the accuracy or adequacy of the disclosures in the Prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is July 23, 2021.

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 19, 2021

Hydrofarm Holdings Group, Inc.

(Exact name of registrant as specified in its charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligations 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 x

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 19, 2021 (the “Redemption Date”), Hydrofarm Holdings Group, Inc. (the “Company”) completed its previously announced redemption (the “Redemption”) of certain of its outstanding warrants (the “Investor Warrants”) to purchase shares of the Company’s common stock that were issued in connection with a private placement of units, each consisting of a share of common stock and an Investor Warrant to purchase an additional one-half (1/2) shares of common stock which concluded on October 30, 2018. Prior to the Redemption Date 3,367,647 Investor Warrants were exercised, generating approximately $56.8 million of gross proceeds to the Company. Pursuant to the Redemption, the Company redeemed 1,491 Investor Warrants for a redemption price of $0.00033712 per Investor Warrant. As of July 20, 2021, the Company had outstanding 43,926,486 shares of common stock.

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.