Filed Pursuant to Rule 424(b)(3)

Registration No. 333-260534

PROSPECTUS SUPPLEMENT NO. 4

(to prospectus dated November 5, 2021)

ALGOMA STEEL GROUP INC.

129,836,439 Common Shares

604,000 Warrants to Purchase Common Shares

24,179,000 Common Shares Underlying Warrants

This prospectus supplement amends and supplements the prospectus dated November 5, 2021, as supplemented or amended from time to time (the “Prospectus”), which forms a part of our Registration Statement on Form F-1 (Registration Statement No. 333-260534). This prospectus supplement is being filed to update and supplement the information included or incorporated by reference in the Prospectus with the information contained in our Report on Form 6-K, furnished to the Securities and Exchange Commission on January 10, 2022 (the “Form 6-K”). Accordingly, we have attached the Form 6-K to this prospectus supplement.

This prospectus supplement updates and supplements the information in the Prospectus and is not complete without, and may not be delivered or utilized except in combination with, the Prospectus, including any amendments or supplements thereto. This prospectus supplement should be read in conjunction with the Prospectus and if there is any inconsistency between the information in the Prospectus and this prospectus supplement, you should rely on the information in this prospectus supplement.

Our Common Shares and Warrants are listed on The Nasdaq Stock Market (“Nasdaq”) under the symbols “ASTL” and “ASTLW”, respectively, and on the Toronto Stock Exchange (the “TSX”) under the symbols “ASTL” and “ASTL.WT,” respectively. On January 7, 2022, the last reported sales prices of the Common Shares on Nasdaq and the TSX were $10.32 and C$12.97, respectively, and the last reported sales prices of the Warrants were $2.66 and C$3.39, respectively.

We are a “foreign private issuer” as defined in the U.S. Securities Exchange Act of 1934, as amended (the “Exchange Act”), and are exempt from certain rules under the Exchange Act that impose certain disclosure obligations and procedural requirements for proxy solicitations under Section 14 of the Exchange Act. In addition, our officers, directors and principal shareholders are exempt from the reporting and “short-swing” profit recovery provisions under Section 16 of the Exchange Act. Moreover, we are not required to file periodic reports and financial statements with the U.S. Securities and Exchange Commission as frequently or as promptly as U.S. companies whose securities are registered under the Exchange Act. Additionally, Nasdaq rules allow foreign private issuers to follow home country practices in lieu of certain of Nasdaq’s corporate governance rules. As a result, our shareholders may not have the same protections afforded to shareholders of companies that are subject to all Nasdaq corporate governance requirements.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” beginning on page 7 of the Prospectus, and under similar headings in any amendment or supplements to the Prospectus.

None of the Securities and Exchange Commission, any state securities commission or the securities commission of any Canadian province or territory has approved or disapproved of the securities offered by this prospectus supplement or the Prospectus 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, 2022.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of January 2022

Commission File Number 001-40924

ALGOMA STEEL GROUP INC.

(Exact name of Registrant as specified in its charter)

N/A

(Translation of Registrant’s name)

105 West Street

Sault Ste. Marie, Ontario

P6A 7B4, Canada

(705) 945-2351

(Address and telephone number of registrant’s principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F  ☒            Form 40-F  ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):  ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):  ☐

DOCUMENTS INCLUDED AS PART OF THIS REPORT

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, thereunto duly authorized.

Exhibit 99.1

MEDIA RELEASE

January 10, 2022

Algoma Steel Provides Fiscal 2022 Third Quarter Update

Expects all 37.5 million shares to be issued pursuant to Earnout rights

Financial results expected to be in line with previous guidance

SAULT STE. MARIE, ONTARIO (January 10, 2022) – Algoma Steel Group Inc. (NASDAQ: ASTL; TSX: ASTL) (“Algoma”), a leading Canadian producer of hot and cold rolled steel sheet and plate products, today announced that based on the current estimates of Algoma’s Earnout Adjusted EBITDA (as defined in the Merger Agreement with Legato Merger Corporation) for calendar year-end 2021, it is currently expected that all 37.5 million Algoma common shares will be issued pursuant to the Earnout Rights as prescribed in the Merger Agreement.

“We were pleased with the Company’s performance during the quarter, with expected financial results in line with our previous fiscal third quarter Adjusted EBITDA guidance, albeit on lower than planned shipments of approximately 550,000 tons, as compared to the previously issued guidance of 590,000 to 610,000 tons. Actual shipments were impacted by various issues including increased holiday shutdowns by customers, logistical supply chain constraints, and COVID-related challenges. As a result, our steel inventory has increased, and we expect to ship these deferred tons over the next two fiscal quarters. Algoma’s high percentage of contract business offers stability during periods of volatility and this, combined with the strong demand and improving pricing we are seeing for our plate products, has positioned us well to continue delivering strong results to our shareholders,” said Michael McQuade, Algoma’s Chief Executive Officer.

About Algoma Steel Inc.

Based in Sault Ste. Marie, Ontario, Canada, Algoma is a fully integrated producer of hot and cold rolled steel products including sheet and plate. With a current raw steel production capacity of an estimated 2.8 million tons per year, Algoma’s size and diverse capabilities enable it to deliver responsive, customer-driven product solutions straight from the ladle to direct applications in the automotive, construction, energy, defense, and manufacturing sectors. Algoma is a key supplier of steel products to customers in Canada and Midwest USA and is the only producer of plate steel products in Canada. Algoma’s mill is one of the lowest cost producers of hot rolled sheet steel (HRC) in North America owing in part to its state-of-the-art Direct Strip Production Complex (“DSPC”), which is the newest thin slab caster in North America with direct coupling to a basic oxygen furnace (BOF) melt shop.

Algoma Steel Inc. | 105 West Street, Sault Ste. Marie, ON, Canada P6A 7B4 | T: 705-945-2351 F: 705-945-2203 | algoma.com

YOUR PARTNER IN STEEL. SINCE 1901

Algoma has achieved several meaningful improvements over the last several years that are expected to result in enhanced long-term profitability for the business. Algoma has upgraded its DSPC facility and recently installed its No. 2 Ladle Metallurgy Furnace. Additionally, Algoma has cost cutting initiatives underway and is in the process of modernizing its plate mill facilities.

Today Algoma is returning to its roots as a customer-focused, entrepreneurial company with the courage and growing capability to meet the industry’s challenges head-on. It is investing in its people and processes, optimizing and modernizing so that it will continue to be your partner in steel.

Cautionary Statement Regarding Forward-Looking Statements

This news release contains “forward-looking information” under applicable Canadian securities legislation and “forward-looking statements” within the meaning of the U.S. Private Securities Litigation Reform Act of 1995 (collectively, “forward looking statements”). These forward-looking statements generally are identified by the words “believe,” “project,” “expect,” “anticipate,” “estimate,” “intend,” “strategy,” “future,” “opportunity,” “plan,” “pipeline,” “may,” “should,” “will,” “would,” “will be,” “will continue,” “will likely result,” and similar expressions, and include statements regarding the company’s expected Adjusted EBITDA, the issuance of shares pursuant to the Earnout Rights, the company’s ability to ship additional steel inventory over the next two fiscal quarters and that the timing of such shipments are not expected to impact realized price per ton. Forward-looking statements are predictions, projections and other statements about future events that are based on current expectations and assumptions. Many factors could cause actual future events to differ materially from the forward-looking statements in this document, including but not limited to: the risk that the benefits of the recently completed merger may not be realized; the risks that Algoma will be unable to realize its business plans, including its proposed transformation journey and development of its supply chain; the risks associated with the steel industry generally; and changes in general economic conditions, including as a result of the COVID-19 pandemic. The foregoing list of factors is not exhaustive and readers should also consider the other risks and uncertainties set forth in the section entitled “Risk Factors” and “Cautionary Note Regarding Forward-Looking Statements” in Algoma’s public filings, including the registration statement on Form S-1 filed by Algoma with the Securities and Exchange Commission and the prospectus filed with the Ontario Securities Commission. Forward-looking statements speak only as of the date they are made. Readers are cautioned not to put undue reliance on forward-looking statements, and Algoma assumes no obligation and does not intend to update or revise these forward-looking statements, whether as a result of new information, future events, or otherwise.

Algoma Steel Inc. | 105 West Street, Sault Ste. Marie, ON, Canada P6A 7B4 | T: 705-945-2351 F: 705-945-2203 | algoma.com

YOUR PARTNER IN STEEL. SINCE 1901

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For more information, please contact:

Mike Moraca

Director - Treasury

Algoma Steel Inc.

Phone: 705.945.3300

E-mail: IR@algoma.com

Algoma Steel Inc. | 105 West Street, Sault Ste. Marie, ON, Canada P6A 7B4 | T: 705-945-2351 F: 705-945-2203 | algoma.com

YOUR PARTNER IN STEEL. SINCE 1901

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