Filed pursuant to Rule 424(b)(3)

Registration No. 333-256938

PROSPECTUS SUPPLEMENT NO. 8

(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 October 26, 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 October 27, 2021, the closing price of our common stock was $32.04 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 October 28, 2021.