SeaStar Medical updates S-3 ATM capacity within I.B.6 float limits

Rhea-AI Filing Summary

SeaStar Medical Holding Corporation is registering the offer and sale of up to $1,901,000 of its common stock under an at-the-market program with H.C. Wainwright & Co. acting as sales agent. The company previously established an ATM facility with an aggregate offering price of $2,279,989, of which $1,170,258 of common stock had been sold as of November 6, 2025. This supplement amends the prior prospectus to align the maximum amount that may be sold with the limits of General Instruction I.B.6 of Form S-3, which currently permits offerings up to an aggregate price of $1,901,390. SeaStar’s public float held by non-affiliates is stated as $44,394,521, based on 35,802,033 shares at $1.24 per share, and the company notes it has sold $12,896,813 of common stock under these I.B.6 limits over the prior 12-month period.

Insights

Capital Markets Analyst

SeaStar increases at-the-market capacity within Form S-3 I.B.6 limits.

SeaStar Medical Holding Corporation updates its at-the-market equity program with H.C. Wainwright & Co. by registering the offer and sale of up to $1,901,000 of common stock. This sits under an existing ATM framework that originally allowed up to $2,279,989 in aggregate sales, of which $1,170,258 had already been issued as of November 6, 2025.

The supplement is structured to comply with General Instruction I.B.6 of Form S-3, which caps primary offerings at one-third of public float while that float remains below $75,000,000. SeaStar reports a public float of $44,394,521, derived from 35,802,033 non-affiliate shares at $1.24 per share as of the referenced dates, and discloses that it has sold $12,896,813 of common stock under these I.B.6 limits in the prior 12-month period.

For holders, this filing mainly formalizes additional flexibility for at-the-market issuance rather than announcing a specific transaction. Any actual dilution or capital raised will depend on future share sales under the ATM and prevailing market prices, including the recent Nasdaq closing price of $0.3031 per share on November 17, 2025.

Filed Pursuant to Rule 424(b)(5)
Registration No. 333-275968

PROSPECTUS SUPPLEMENT
(To Prospectus dated December 22, 2023)

SeaStar Medical Holding Corporation

Up to $1,901,000 of Common Stock

We previously entered into an at the market offering agreement (the “Offering Agreement”) with H.C. Wainwright & Co., LLC (“Wainwright”) pursuant to which we may offer and sell shares of our common stock from time to time through Wainwright, acting as sales agent, under our prospectus supplement, dated October 14, 2025, and accompanying base prospectus (collectively, the “Prospectus”), having an aggregate offering price of up to $2,279,989. As of November 6, 2025, we have sold $1,170,258 of our common stock pursuant to the Offering Agreement, which leaves $1,109,731 of common stock available under the Prospectus. This prospectus supplement should be read in conjunction with the Prospectus, and is qualified by reference thereto, except to the extent that the information herein amends or supersedes the information contained in the Prospectus. This prospectus supplement is not complete without, and may only be delivered or utilized in connection with, the Prospectus, and any future amendments or supplements thereto.

We are filing this prospectus supplement to amend the Prospectus to increase the maximum amount of shares of our common stock that we are eligible to sell under the Offering Agreement pursuant to the Prospectus under General Instruction I.B.6. of Form S-3. As a result of these limitations, we may currently offer and sell shares of our common stock having an aggregate offering price of up to $1,901,390. Pursuant to this prospectus supplement, we are registering the offer and sale of up to $1,901,000 of shares of our common stock. In the event that our public float increases above $75.0 million, we will no longer be subject to the limitations of General Instruction I.B.6 of Form S-3.

The aggregate market value of our outstanding common stock held by non-affiliates is $44,394,521, based on 35,802,033 shares of outstanding common stock held by non-affiliates as of November 17, 2025 and a price per share of $1.24, the closing price of our common stock on September 24, 2025. Pursuant to General Instruction I.B.6 of Form S-3, in no event will we offer to sell, pursuant to the registration statement of which this prospectus supplement forms a part, securities in a public primary offering with a value exceeding one-third of our public float in any 12-month calendar period so long as the aggregate market value of our outstanding common stock held by non-affiliates remains below $75 million. During the prior 12-calendar-month period that ends on, and includes, the date of this prospectus supplement, we have sold $12,896,813 of shares of common stock pursuant to General Instruction I.B.6 of Form S-3.

Our common stock is listed on The Nasdaq Capital Market under the symbol “ICU.” On November 17, 2025, the last reported sale price of our common stock on The Nasdaq Capital Market was $0.3031 per share.

Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 3 of the Prospectus, the “Risk Factors” section in our most recent Annual Report on Form 10-K, which is incorporated by reference into this prospectus supplement and the Prospectus, and under similar headings in the other documents that are filed after the date hereof and incorporated by reference into this prospectus supplement and the Prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus supplement or the Prospectus. Any representation to the contrary is a criminal offense.

H.C. Wainwright & Co.

The date of this prospectus supplement is November 20, 2025.

FAQ

What is SeaStar Medical (ICU) registering in this prospectus supplement?

SeaStar Medical is registering the offer and sale of up to $1,901,000 of its common stock under an at-the-market offering program with H.C. Wainwright & Co.

How does this supplement change SeaStar Medical (ICU)'s existing at-the-market program?

The supplement amends the prior prospectus to increase the maximum amount of common stock SeaStar may sell under the ATM so that it can offer shares with an aggregate price of up to $1,901,390, consistent with Form S-3 General Instruction I.B.6 limits.

How much stock has SeaStar Medical (ICU) already sold under the ATM agreement?

As of November 6, 2025, SeaStar had sold $1,170,258 of its common stock under the at-the-market offering agreement with H.C. Wainwright & Co.

What is SeaStar Medical (ICU)'s current public float and share count used for the I.B.6 limit?

The company reports a public float of $44,394,521, based on 35,802,033 shares of common stock held by non-affiliates as of November 17, 2025 at a price of $1.24 per share.

How much has SeaStar Medical (ICU) sold under Form S-3 General Instruction I.B.6 in the last 12 months?

Over the prior 12-calendar-month period ending on the date of the supplement, SeaStar states it has sold $12,896,813 of common stock pursuant to General Instruction I.B.6 of Form S-3.

What is the recent trading price of SeaStar Medical (ICU) common stock?

On November 17, 2025, the last reported sale price of SeaStar Medical's common stock on The Nasdaq Capital Market was $0.3031 per share.

Under what condition will SeaStar Medical (ICU) no longer be subject to Form S-3 I.B.6 limits?

The company notes that if its public float increases above $75.0 million, it will no longer be subject to the one-third public float limitation in General Instruction I.B.6 of Form S-3.