AltaGas Ltd. — Share Issue/Capital Change 2025

Nov 5, 2025

No securities regulatory authority has expressed an opinion about these securities and it is an offence to claim otherwise.

This prospectus supplement (this “Prospectus Supplement”), together with the short form base shelf prospectus dated March 12, 2025 to which it relates, as amended or supplemented (the “Prospectus”), and each document incorporated, or deemed to be incorporated, by reference into the Prospectus, constitutes a public offering of these securities only in those jurisdictions where they may be lawfully offered for sale and therein only by persons permitted to sell such securities.

The securities offered hereunder have not been, and will not be, registered under the United States Securities Act of 1933, as amended (the “U.S. Securities Act”), or any U.S. state securities laws and may not be offered, sold or delivered, directly or indirectly, within the United States of America or to, or for the account or benefit of, U.S. persons (as such term is defined in Regulation S under the U.S. Securities Act), except as permitted by the Underwriting Agreement (as defined herein) and pursuant to an exemption from the registration requirements of the U.S. Securities Act and applicable U.S. state securities laws. This Prospectus Supplement does not constitute an offer to sell or a solicitation of an offer to buy any of the securities offered hereunder within the United States of America. See “Plan of Distribution”.

Information has been incorporated by reference into the Prospectus from documents filed with securities commissions or similar authorities in Canada. Copies of the documents incorporated therein by reference may be obtained on request without charge from the Executive Vice President and Chief Legal Officer of AltaGas Ltd., at 1300, 707 – 5th Street SW, Calgary, Alberta, T2P 1V8, Telephone (403) 691-7575, and are also available electronically at www.sedarplus.ca.

PROSPECTUS SUPPLEMENT
To a Short Form Base Shelf Prospectus Dated March 12, 2025

New Issue

November 5, 2025

AltaGas

ALTAGAS LTD.

$400,465,000

10,100,000 Common Shares

This Prospectus Supplement, together with the Prospectus, qualifies the distribution (the “Offering”) of 10,100,000 common shares (the “Offered Shares”) in the share capital of AltaGas Ltd. (“AltaGas” or the “Corporation”) at a price of $39.65 (the “Offering Price”) per Offered Share.

The Offering is being made pursuant to an underwriting agreement dated November 5, 2025 (the “Underwriting Agreement”) among the Corporation and a syndicate of underwriters led by CIBC World Markets Inc. (“CIBC”), TD Securities Inc. (“TD”), RBC Dominion Securities Inc. (“RBC”) and Scotia Capital Inc. (“Scotia” and, together with CIBC, TD and RBC, the “Joint Bookrunners”), and including BMO Nesbitt Burns Inc. (“BMO”), National Bank Financial Inc. (“NBF”), Merrill Lynch Canada Inc. (“Merrill”), Wells Fargo Securities Canada, Ltd. (“Wells”), Mizuho Securities Canada Inc. (“Mizuho”), ATB Securities Inc. (“ATB”), Peters & Co. Limited, J.P. Morgan Securities Canada Inc. (“JPM”), Morgan Stanley Canada Limited and Tudor, Pickering, Holt & Co. Securities – Canada ULC (collectively, the “Underwriters”). The Offering is being made under this Prospectus in each of the provinces of Canada. In addition, the Underwriters may offer the Offered Shares on a private placement basis outside of Canada in compliance with applicable local securities laws and the terms of the Underwriting Agreement.

The Corporation’s common shares (the “Common Shares”) are listed and posted for trading on the Toronto Stock Exchange (the “TSX”) under the symbol “ALA”. The closing price of the Common Shares on the TSX on November 3, 2025, the last full trading day prior to the announcement of the Offering, was $41.02. The closing price of the Common Shares on the TSX on November 4, 2025, the last trading day prior to the filing of this Prospectus Supplement, was $40.27. The TSX has conditionally approved the listing of the Offered Shares on the TSX. Listing is subject to the Corporation fulfilling all of the listing requirements of the TSX on or before February 3, 2026.

Offering Price: $39.65 per Common Share

	Price to the Public(1)	Underwriters’ Fee	Net Proceeds to the Corporation(2)
Per Offered Share	$39.65	$1.586	$38.064
Total(3)	$400,465,000	$16,018,600	$384,446,400

Notes:

(1) The Offering Price was established by arm’s length negotiations between the Corporation and the Joint Bookrunners, on behalf of the Underwriters, with reference to the market price of the Common Shares and other factors.

(2) Before deducting expenses of the Offering estimated at approximately $1,000,000 (exclusive of all applicable taxes), which, together with the Underwriters’ Fee, will be paid from the proceeds of the Offering.

(3) The Corporation has granted to the Underwriters an option (the “Over-Allotment Option”), exercisable in whole or in part at any time up to 30 days after the Closing Date (as defined herein), to purchase up to an additional 1,515,000 Common Shares (the “Additional Shares”) at the Offering Price and on the same terms as set forth above, to cover the Underwriters’ over-allocation position, if any, and for market stabilization purposes. If the Over-Allotment Option is exercised in full, the total Price to the Public, the Underwriters’ Fee and the Net Proceeds to the Corporation (before deducting expenses of the Offering) will be $460,534,750, $18,421,390 and $442,113,360, respectively. This Prospectus Supplement, together with the Prospectus, qualifies the grant of the Over-Allotment Option and the distribution of any Additional Shares upon exercise of the Over-Allotment Option. Unless the context otherwise requires, all references to “Offered

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Shares" in this Prospectus Supplement include references to Additional Shares that may be issued pursuant to the Over-Allotment Option. A purchaser who acquires Additional Shares forming part of the Underwriters' over-allocation position acquires those Additional Shares under this Prospectus Supplement, together with the Prospectus, regardless of whether the over-allocation position is ultimately filled through the exercise of the Over-Allotment Option or secondary market purchases. See "Plan of Distribution".

Underwriters' Position	Maximum Size or Number of Common Shares Available	Exercise Period	Exercise Price
Over-Allotment Option	Option to purchase up to 1,515,000 Common Shares	At any time up to 30 days following Closing	Offering Price

The Underwriters, as principals, conditionally offer the Offered Shares, subject to the prior sale, if, as and when issued, sold and delivered by the Corporation and accepted by the Underwriters in accordance with the conditions contained in the Underwriting Agreement and subject to the approval of certain legal matters on behalf of the Corporation by Torys LLP and on behalf of the Underwriters by Blake, Cassels & Graydon LLP. See “Plan of Distribution”.

Subject to applicable laws, the Underwriters may, in connection with the Offering, over-allot or effect transactions intended to stabilize or maintain the market price of the Common Shares at levels other than those that might otherwise prevail on the open market. Such transactions, if commenced, may be discontinued at any time. The Underwriters propose to offer the Offered Shares initially at the Offering Price. After the Underwriters have made reasonable efforts to sell all of the Offered Shares at the Offering Price, the Underwriters may subsequently reduce the selling price to investors from time to time in order to sell any of the Offered Shares remaining unsold. Any such reduction will not affect the proceeds received by the Corporation. See “Plan of Distribution”.

CIBC, TD, RBC, Scotia, BMO, NBF, Merrill, Wells, Mizuho and JPM are, directly or indirectly, subsidiaries or affiliates of Canadian chartered banks or other financial institutions that are lenders to AltaGas or its subsidiaries. In addition, ATB is a subsidiary of ATB Financial. ATB Financial is an affiliate of Alberta Treasury Branches, which is a provincially regulated financial institution that is also a lender to AltaGas or its subsidiaries. A portion of the net proceeds from the sale of the Offered Shares may be used to reduce the indebtedness of AltaGas to such lenders. Accordingly, pursuant to applicable securities legislation, AltaGas may be considered a "connected issuer" of such Underwriters. See "Plan of Distribution - Relationships Between Certain of the Underwriters and AltaGas" and "Use of Proceeds".

An investment in the Offered Shares involves certain risks that should be considered by a prospective purchaser. See "Risk Factors" in this Prospectus Supplement and in the Prospectus. You should rely only on the information contained or incorporated by reference in this Prospectus Supplement and the Prospectus. AltaGas has not authorized anyone to provide you with information different from that which is contained in this Prospectus Supplement and the Prospectus and the information contained in any documents incorporated by reference is accurate only as of the date of that document. Prospective investors should carefully consider the foregoing risk factors before purchasing any Offered Shares.

Subscriptions for Offered Shares will be received subject to rejection or allocation, in whole or in part, and the Underwriters reserve the right to close the subscription books at any time without notice. Registrations and transfers of Offered Shares will be effected electronically through the non-certificated inventory system ("NCI") administered by CDS Clearing and Depository Services Inc. ("CDS"). No certificates evidencing the Offered Shares will be issued to purchasers of the Offered Shares. AltaGas expects that purchasers of Offered Shares will receive only a customer confirmation or statement from the Underwriter or other registered dealer which is a CDS participant from or through which an Offered Share is purchased. See "Plan of Distribution - Non-Certificated Inventory System".

Closing of the Offering ("Closing") is expected to occur on or about November 7, 2025, or such other date as the Corporation and the Underwriters may agree (such actual closing date hereinafter referred to as the "Closing Date"). In any event, the Offered Shares are to be taken up by the Underwriters, if at all, on or before a date not later than December 15, 2025. See "Plan of Distribution".

Subject to certain conditions set out under "Eligibility for Investment", the Offered Shares will constitute a qualified investment for trusts governed by DPSPs, RRSPs, RRIFs, RESPs, RDSPs, TFSAs and FHSAs (as such terms are defined herein). As set out under "Eligibility for Investment", prospective holders of Offered Shares who intend to hold their Offered Shares in an Exempt Plan (as defined herein) should consult their own advisors regarding their particular circumstances.

Investors should be aware that the acquisition, holding and disposition of the Offered Shares may have tax consequences in Canada or elsewhere depending on each particular investor's specific circumstances. Investors should consult their own tax advisors with respect to such tax considerations. Investors who are not residents of Canada for tax purposes should consult their own tax advisors concerning the consequences to them of acquiring Offered Shares under the Offering.

Information about the right to withdraw or rescind from an agreement to purchase securities is provided under the heading "Purchasers' Statutory Rights".

All references in this Prospectus Supplement to "Canada" mean Canada, its provinces, its territories, its possessions and all areas subject to its jurisdiction, and all references to “$” or “Canadian dollars” mean the lawful currency of Canada. In this Prospectus Supplement, all references to “U.S.” or “United States” mean the United States of America, its states, its territories, its possessions and all areas subject to its jurisdiction, and all references to “US$”, “US dollars” or “United States dollars” mean the lawful currency of the United States. In this Prospectus Supplement, unless otherwise specified, all dollar amounts are expressed in Canadian dollars.

AltaGas' registered and head office is located at 1300, 707 - 5th Street SW, Calgary, Alberta, T2P 1V8.

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TABLE OF CONTENTS

Page

IMPORTANT NOTICE ABOUT INFORMATION IN THIS PROSPECTUS SUPPLEMENT AND THE PROSPECTUS...4
DOCUMENTS INCORPORATED BY REFERENCE...4
MARKETING MATERIALS...5
NOTE REGARDING FORWARD–LOOKING STATEMENTS...5
RECENT DEVELOPMENTS...7
CAPITALIZATION OF ALTAGAS...7
USE OF PROCEEDS...7
PLAN OF DISTRIBUTION...8
PRIOR SALES...11
PRICE RANGE AND TRADING VOLUME OF THE COMMON SHARES...11
DESCRIPTION OF COMMON SHARES...11
DIVIDENDS...11
ELIGIBILITY FOR INVESTMENT...12
RISK FACTORS...12
INTERESTS OF EXPERTS...13
TRANSFER AGENT AND REGISTRAR...13
ENFORCEMENT OF JUDGMENTS AGAINST FOREIGN PERSONS OR COMPANIES...13
PURCHASERS’ STATUTORY RIGHTS...13
CERTIFICATE OF THE UNDERWRITERS...C-1

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IMPORTANT NOTICE ABOUT INFORMATION IN THIS PROSPECTUS SUPPLEMENT AND THE PROSPECTUS

This document consists of two parts. The first part of this document is this Prospectus Supplement, which describes the specific terms of the Common Shares that AltaGas is qualifying for distribution and also adds to, and updates certain information contained in, the Prospectus and the documents incorporated by reference therein. The second part of this document is the Prospectus, which gives more general information, some of which may not apply to the Common Shares that AltaGas is qualifying for distribution. Defined terms or abbreviations used in this Prospectus Supplement that are not defined herein have the meanings ascribed thereto in the Prospectus.

If the description of the Common Shares or any other information varies between this Prospectus Supplement, the Prospectus and the documents incorporated by reference therein, you should rely on the information in this Prospectus Supplement.

AltaGas and the Underwriters have not authorized anyone to provide you with information other than that contained in this Prospectus Supplement or contained in, or incorporated by reference into, the Prospectus or to make any representations other than those contained in this Prospectus Supplement or contained in, or incorporated by reference into, the Prospectus. AltaGas and the Underwriters take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you or any representation that others may make to you. AltaGas is not, and the Underwriters are not, making an offer to sell the Common Shares in any jurisdiction where such offer or sale is not permitted. You should assume that the information in this Prospectus Supplement and the Prospectus, as well as the information in any document incorporated by reference into the Prospectus that AltaGas previously filed with any securities commission or similar regulatory authority in Canada, is accurate only as of the respective dates of the applicable documents. AltaGas' business, financial condition, results of operations and prospects may have changed since those dates. A prospective purchaser should carefully read this Prospectus Supplement and the Prospectus and the documents incorporated by reference therein and consult their own professional advisors to assess the risks associated with, and the income tax, legal, and other aspects of, an investment in the Common Shares.

DOCUMENTS INCORPORATED BY REFERENCE

This Prospectus Supplement is deemed to be incorporated by reference into the Prospectus solely for the purposes of the Offering. Other documents are also incorporated or deemed to be incorporated by reference into the Prospectus and reference should be made to the Prospectus for full particulars.

See “Documents Incorporated by Reference” in the Prospectus. The following documents are specifically incorporated by reference in and form an integral part of the Prospectus:

(a) the annual information form of AltaGas dated March 6, 2025 for the year ended December 31, 2024 (the “AIF”);

(b) the audited annual consolidated financial statements of AltaGas as at and for the years ended December 31, 2024 and December 31, 2023, together with the notes thereto and the auditors’ report thereon;

(c) the management’s discussion and analysis of the financial and operating results of AltaGas for the year ended December 31, 2024 (the “Annual MD&A”);

(d) the unaudited interim condensed consolidated financial statements of AltaGas as at and for the three and nine months ended September 30, 2025, together with the notes thereto;

(e) the management’s discussion and analysis of the financial and operating results of AltaGas for the three and nine months ended September 30, 2025 (the “Interim MD&A”);

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(f) the management information circular of AltaGas dated March 6, 2025 relating to the annual and special meeting of holders of Common Shares held on May 1, 2025;
(g) the investor presentation prepared for potential investors in connection with the Offering dated and filed on November 3, 2025; and
(h) the term sheet prepared for potential investors in connection with the Offering dated and filed on November 3, 2025.

Any documents of the type required by National Instrument 44-101 – Short Form Prospectus Distributions to be incorporated by reference into a short form prospectus, including any material change reports (excluding material change reports filed on a confidential basis), interim financial reports, annual financial statements and the auditors’ reports thereon, management’s discussion and analysis of financial condition and results of operations, information circulars, annual information forms and business acquisition reports, as well as all prospectus supplements disclosing additional or updated information relating to the Offering filed by the Corporation with the securities commissions or similar authorities in each of the provinces of Canada subsequent to the date of this Prospectus Supplement and prior to the termination of the Offering, shall be deemed to be incorporated by reference into the Prospectus. These documents will be available through the internet on SEDAR+, which can be accessed at www.sedarplus.ca.

Any statement contained in the Prospectus, in this Prospectus Supplement or in any other document (or part thereof) incorporated or deemed to be incorporated by reference into the Prospectus shall be deemed to be modified or superseded, for purposes of this Prospectus Supplement, to the extent that a statement contained herein or in any other subsequently filed document (or part thereof) which also is or is deemed to be incorporated by reference into the Prospectus modifies or supersedes such statement. The modifying or superseding statement need not state that it has modified or superseded a prior statement or include any other information set forth in the document which it modifies or supersedes. The making of a modifying or superseding statement shall not be deemed an admission for any purposes that the modified or superseded statement, when made, constituted a misrepresentation, an untrue statement of a material fact or an omission to state a material fact that is required to be stated or that is necessary to make a statement not misleading in light of the circumstances in which it was made. Any statement so modified or superseded shall not be deemed to constitute a part of this Prospectus Supplement or the Prospectus, except as so modified or superseded.

MARKETING MATERIALS

“Template versions” of the “marketing materials” (as such terms are defined in National Instrument 41-101 – General Prospectus Requirements) in respect of Offering are incorporated by reference into this Prospectus Supplement but are not part of this Prospectus Supplement to the extent that their contents have been modified or superseded by a statement contained in this Prospectus Supplement. In addition, any template version of any other marketing materials filed with the securities commission or similar authority in each of the provinces of Canada in connection with the Offering after the date hereof but prior to the termination of the distribution of the Offered Shares under this Prospectus Supplement is deemed to be incorporated by reference herein.

NOTE REGARDING FORWARD-LOOKING STATEMENTS

This Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus contain “forward-looking information” and “forward-looking statements” within the meaning of Canadian securities laws (collectively, “forward-looking statements”).

In addition to the following cautionary statement and the cautionary statement contained under “Note Regarding Forward-Looking Statements” in the Prospectus, with respect to forward-looking statements contained in the documents incorporated by reference into the Prospectus, prospective purchasers should refer to “Forward-Looking Information and Statements” in the AIF and the face page of each of the Annual MD&A and the Interim MD&A, as well as the advisories section of any documents incorporated by reference into the Prospectus that are filed after the date of this Prospectus Supplement and prior to the termination of the Offering.

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When used in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus, the words “may”, “can”, “would”, “could”, “should”, “will”, “intend”, “plan”, “anticipate”, “believe”, “aim”, “seek”, “propose”, “contemplate”, “estimate”, “focus”, “strive”, “forecast”, “expect”, “project”, “target”, “potential”, “objective”, “continue”, “outlook”, “vision”, “opportunity” and similar expressions suggesting future events or future performance, as they relate to AltaGas or an affiliate of AltaGas, are intended to identify forward-looking statements. In particular, this Prospectus Supplement contains forward-looking statements pertaining to: the anticipated use of proceeds from the issuance of the Offered Shares; the plan of distribution of the Offered Shares; AltaGas’ plans respecting the declaration and payment of dividends; the listing of the Common Shares on the TSX; and AltaGas’ expectations with respect to MVP (as defined herein). In addition, the Prospectus and the documents incorporated by reference in the Prospectus contain forward-looking statements pertaining to, among other things, AltaGas’ future plans, business objectives, expected growth, business and growth projects, business strategies and the expected results of future operations.

Various factors or assumptions are typically applied by AltaGas in drawing conclusions or making the forecasts, projections, predictions or estimations expressed or implied by forward-looking statements based on information currently available to AltaGas at the time such forward-looking statements were made. The material assumptions in making these forward-looking statements are disclosed in the Prospectus, the AIF, the Annual MD&A and the Interim MD&A, as may be modified or superseded by documents incorporated or deemed to be incorporated by reference in the Prospectus and include, among other assumptions: expected commodity supply, demand and pricing; volumes and rates; exchange rates; inflation; interest rates; credit ratings; regulatory approvals and policies; future operating and capital costs; project completion dates; capacity expectations; effective tax rates; financing initiatives; the outcomes of significant commercial contract negotiation; degree day variance from normal; pension discount rate; the performance of the businesses underlying each sector; impacts of AltaGas’ hedging program; weather; frac spread; access to capital; timing and receipt of regulatory approvals; seasonality; planned and unplanned plant outages; anticipated timing of in-service dates of new projects and of acquisition and divestiture activities; operational expenses; returns on investments; and dividend levels. AltaGas believes that the expectations reflected in the forward-looking statements included in this Prospectus Supplement and those included in, or incorporated by reference into, the Prospectus are reasonable, in each case, as at the time that such statements were made, but no assurance can be given that these expectations will prove to be correct and the forward-looking statements included in this Prospectus Supplement or included in, or incorporated by reference into, the Prospectus should not be unduly relied upon.

Forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause actual results, events and achievements to differ materially from those expressed or implied by such statements, including those risks, uncertainties and other factors discussed under the heading “Note Regarding Forward-Looking Statements” in the Prospectus and the AIF, on the face page of each of the Annual MD&A and Interim MD&A and under the heading “Risk Factors” in the Prospectus, the AIF and this Prospectus Supplement. Many factors could cause AltaGas’ or any of its business segment’s actual results, performance or achievements to vary from those described in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus, including, without limitation, those listed above and the assumptions upon which they are based proving incorrect. These factors should not be construed as exhaustive. Should one or more of these risks or uncertainties materialize, or should assumptions underlying forward-looking statements prove incorrect, actual results may vary materially from those described in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus as intended, planned, anticipated, believed, sought, proposed, estimated, forecasted, expected, projected or targeted and such forward-looking statements included in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus should not be unduly relied upon. The impact of any one assumption, risk, uncertainty or other factor on a particular forward-looking statement cannot be determined with certainty because they are interdependent, and AltaGas’ future decisions and actions will depend on management’s assessment of all information at the relevant time. Such statements speak only as of the date of this Prospectus Supplement, the Prospectus or as of the date specified in the documents incorporated by reference in the Prospectus, as the case may be. AltaGas does not intend, and does not assume any obligation, to update these forward-looking statements except as required by law. The forward-looking statements contained in this Prospectus Supplement, the Prospectus and the documents incorporated by reference in the Prospectus are expressly qualified by these cautionary statements.

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Financial outlook information contained in the Prospectus and the documents incorporated by reference in the Prospectus about prospective results of operations, financial position or cash flow is based on assumptions about future events, including economic conditions and proposed courses of action, based on management’s assessment of the relevant information available as of the date of the Prospectus or as of the date specified in the documents incorporated by reference therein, as the case may be. Readers are cautioned that such financial outlook information contained in the Prospectus and the documents incorporated by reference in the Prospectus should not be used for purposes other than for which it is disclosed herein or therein, as the case may be.

Neither AltaGas’ independent auditors nor any other independent account has compiled, reviewed, examined or performed any other assurance procedures, or expressed any form of assurance with respect to the financial outlook information included in this Prospectus Supplement. The report of AltaGas’ independent auditors incorporated by reference in the Prospectus relates to AltaGas’ historical audited financial statements and does not extend to the unaudited financial outlook information and should not be read to do so.

RECENT DEVELOPMENTS

On November 3, 2025, concurrently with the announcement of the Offering, AltaGas announced its decision to retain ownership in the Mountain Valley Pipeline (“MVP”), including the MVP Mainline, MVP Boost, and MVP Southgate projects, as a long-term investment. The decision to keep MVP was made having consideration to a number of recent developments, including with respect to the performance levels of the MVP Mainline and MVP Boost, progress made on the MVP Southgate project and anticipated MVP Boost project-level returns.

CAPITALIZATION OF ALTAGAS

Other than: (i) additional borrowings of an aggregate of approximately $155 million under the Syndicated Facility (as defined herein); (ii) additional issuances of approximately US$66 million of commercial paper; (iii) additional borrowings of an aggregate of approximately US$47.5 million under the Revolving Facility; and (iv) the issuance of the Offered Shares and the expected use of the net proceeds therefrom as described under “Use of Proceeds”, there has been no material change in the share and loan capital of the Corporation, on a consolidated basis, since September 30, 2025.

As at September 30, 2025, after giving effect to the foregoing, the consolidated loan capital of AltaGas would be approximately $10,377 million and its common share capital would be comprised of 309,586,164 Common Shares outstanding. See “Recent Developments” and “Use of Proceeds”. See also AltaGas’ Annual MD&A and Interim MD&A, each of which are incorporated by reference in this Prospectus Supplement.

USE OF PROCEEDS

The net proceeds of the Offering and the net proceeds of the Over-Allotment Option, if any, will be used to pay down existing indebtedness including, without limitation, indebtedness under AltaGas’ credit facilities and to fund future growth projects. The indebtedness incurred under AltaGas’ credit facilities was incurred in the normal course of business to provide working capital in respect of ongoing operations and for other general corporate purposes.

The use of proceeds by AltaGas noted above is consistent with AltaGas’ overall business objective, corporate strategy and major initiatives supporting its objective and strategy, which are summarized in the Annual MD&A and the Interim MD&A. There is no particular significant event or milestone that must occur for AltaGas to accomplish its business objectives.

While the Corporation intends to use the net proceeds of the Offering and net proceeds of the Over-Allotment Option, if any, as stated above, there may be circumstances that are not known at this time where a reallocation of the net proceeds may be advisable for business reasons that Management or the Board of Directors of the Corporation believe are in the Corporation’s best interests.

Certain of the Underwriters are, directly or indirectly, subsidiaries or affiliates of Canadian chartered banks or other financial institutions which are lenders to AltaGas or its subsidiaries. Consequently, AltaGas may be considered to be

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a connected issuer of such Underwriters for purposes of applicable securities legislation. See “Plan of Distribution – Relationships Between Certain of the Underwriters and AltaGas”.

PLAN OF DISTRIBUTION

Pursuant to the Underwriting Agreement, the Corporation has agreed to issue and sell and the Underwriters have severally agreed to purchase, as principals, on the Closing Date, subject to compliance with all necessary legal requirements and the terms and conditions stipulated in the Underwriting Agreement, an aggregate of 10,100,000 Offered Shares offered hereby at a price of $39.65 per Offered Share for total gross consideration of $400,465,000, payable in cash to the Corporation (less the Underwriters’ Fee) against delivery of the Offered Shares on the Closing Date. The Offered Shares are being offered to the public in all of the provinces of Canada. The Offering Price was determined by arm’s length negotiations between the Corporation and the Joint Bookrunners, on behalf of the Underwriters, with reference to the then-current market price of the Common Shares and other factors.

The Underwriting Agreement provides that the Corporation will pay the Underwriters an aggregate fee of $16,018,600, representing 4.0% of the gross proceeds from the issuance of the Offered Shares, in consideration for their services in connection with the Offering. The Underwriters’ Fee will be paid to the Underwriters from the proceeds of the Offering on the Closing Date.

The Corporation has granted to the Underwriters the Over-Allotment Option, exercisable in whole or in part at any time up to 30 days after Closing, to purchase up to 1,515,000 Additional Shares at the Offering Price and on the same terms as the Offering, to cover the Underwriters’ over-allocation position, if any, and for market stabilization purposes. This Prospectus Supplement, together with the Prospectus, also qualifies the grant of the Over-Allotment Option and the distribution of any Additional Shares upon exercise of the Over-Allotment Option. A purchaser who acquires Additional Shares forming part of the Over-Allotment Option acquires those Additional Shares under this Prospectus Supplement, together with the Prospectus, regardless of whether the Underwriters’ over-allocation position is ultimately filled through the exercise of the Over-Allotment Option or secondary market purchases.

The Underwriters will also be paid an aggregate fee of 4.0% of the gross proceeds realized from the sale of any Additional Shares sold pursuant to the exercise of the Over-Allotment Option. The Underwriters’ Fee in respect of the Over-Allotment Option will be payable in full upon closing of the Over-Allotment Option.

The obligations of the Underwriters under the Underwriting Agreement are several, not joint nor joint and several. If an Underwriter fails to purchase the Offered Shares that it has agreed to purchase pursuant to the Underwriting Agreement, the other Underwriters may, but are not obligated to, purchase such Offered Shares, provided that, if the number of Offered Shares not purchased is 10% or less of the aggregate number of Offered Shares agreed to be purchased by the Underwriters, then each of the other Underwriters is obligated to purchase severally the Offered Shares not taken up, on a pro rata basis or as they may otherwise agree as among themselves. If the number of Offered Shares not purchased is greater than 10% of the aggregate number of Offered Shares agreed to be purchased by the Underwriters, then each of the other Underwriters shall be relieved of its obligations to purchase its respective percentage of the Offered Shares, subject to the terms and conditions of the Underwriting Agreement. The Underwriters are, however, obligated to take up and pay for all of the Offered Shares included in the base Offering if any of the Offered Shares are purchased under the Underwriting Agreement.

Upon the occurrence of certain stated termination events as set forth in the Underwriting Agreement, each Underwriter is entitled, at its option, to terminate and cancel its obligations under the Underwriting Agreement, without any liability on its part if, prior to the Closing time on the Closing Date.

The TSX has conditionally approved the listing of the Offered Shares on the TSX. Listing will be subject to the Corporation fulfilling all of the listing requirements of the TSX on or before February 3, 2026.

The Underwriters propose to offer the Common Shares initially at the Offering Price. After the Underwriters have made a reasonable effort to sell all of the Common Shares at the Offering Price, the offering price for the Common Shares may be decreased and may be further changed from time to time to an amount not greater than the Offering Price, and the compensation realized by the Underwriters will be decreased by the amount that the aggregate price

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paid by purchasers for the Common Shares is less than the price paid by the Underwriters to the Corporation. Any such reduction will not affect the proceeds received by the Corporation.

Subject to the terms of the Underwriting Agreement, the Corporation has agreed to indemnify the Underwriters and their affiliates and their respective shareholders, directors, officers, partners, employees and agents against certain liabilities and expenses, including in connection with the non-compliance or alleged non-compliance by the Corporation under applicable securities laws, or to contribute to any payments the Underwriters may be required to make in respect thereof.

This Offering is being made in each of the provinces of Canada. The Common Shares have not been, and will not be, registered under the U.S. Securities Act or the securities laws of any state of the United States and may not be offered, sold or delivered, directly or indirectly, in the United States except in accordance with the Underwriting Agreement and pursuant to an exemption from the registration requirements of the U.S. Securities Act and applicable U.S. state securities laws. Each Underwriter has agreed that it will not offer or sell Offered Shares within the United States, except as permitted in the Underwriting Agreement and as expressly permitted by applicable laws of the United States. The Underwriting Agreement provides that the Underwriters may offer and sell the Offered Shares through their U.S. broker-dealer affiliates in the United States to "qualified institutional buyers" (as defined in Rule 144A under the U.S. Securities Act ("Rule 144A")) in accordance with Rule 144A and similar exemptions from registration under applicable U.S. state securities laws. The Underwriting Agreement also provides that the Underwriters may offer and sell the Offered Shares outside the United States in accordance with Regulation S under the U.S. Securities Act. This Prospectus Supplement does not constitute an offer to sell or a solicitation of an offer to buy any of the Offered Shares within the United States.

In addition, until 40 days after the commencement of the Offering, an offer or sale of the Offered Shares within the United States by any dealer (whether or not participating in the Offering) may violate the registration requirements of the U.S. Securities Act if such offer or sale is made otherwise than in accordance with an exemption from registration under the U.S. Securities Act. The Offered Shares sold in the United States will be "restricted securities" within the meaning of Rule 144 under the U.S. Securities Act.

Price Stabilization, Short Positions and Passive Market-Making

Pursuant to policy statements of certain regulators, the Underwriters may not, throughout the period of distribution under this Prospectus Supplement, bid for or purchase the Offered Shares. The foregoing restriction is subject to exceptions, on the condition that the bid or purchase is not engaged in for the purpose of creating actual or apparent active trading in or raising the price of the Offered Shares. These exceptions include a bid or purchase permitted under the Universal Market Integrity Rules of the Canadian Investment Regulatory Organization relating to market stabilization and passive market-making activities and a bid or purchase made for and on behalf of a customer where the order was not solicited during the period of distribution. Under the first-mentioned exception, in connection with the Offering, the Underwriters may over-allot or effect transactions which stabilize or maintain the market price of the Offered Shares at levels other than those which might otherwise prevail in the open market. Those transactions, if commenced, may be discontinued at any time.

Relationships Between Certain of the Underwriters and AltaGas

CIBC, TD, RBC, Scotia, BMO, NBF, Merrill, Wells, Mizuho and JPM are, directly or indirectly, subsidiaries or affiliates of Canadian chartered banks or other financial institutions that are lenders to AltaGas or its subsidiaries. In addition, ATB is a subsidiary of ATB Financial. ATB Financial is an affiliate of Alberta Treasury Branches, which is a provincially regulated financial institution that is also a lender to AltaGas or its subsidiaries.

As at November 5, 2025, AltaGas was indebted in the approximate amount of $1.365 billion to a syndicate of lenders, including lender affiliates of CIBC, TD, RBC, Scotia, BMO, NBF, Merrill, Wells, Mizuho, ATB and JPM under a $2.3 billion revolving term credit facility with a syndicate of Canadian chartered banks (the "Syndicated Facility"). The Syndicated Facility is comprised of a $1.7 billion four-year extendable committed revolving tranche which matures in May 2029 and a $600 million three-year extendable side car revolving tranche which matures in May 2028. The Syndicated Facility is unsecured. AltaGas and its subsidiaries are presently in compliance with the terms of the

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Syndicated Facility and none of the lenders has waived a breach of the agreement governing the Syndicated Facility since its execution. The consolidated financial position of AltaGas has not changed materially since the indebtedness under the Syndicated Facility was incurred.

In addition, as at November 5, 2025, AltaGas was indebted in an approximate amount of US$78.5 million to a syndicate of lenders, including lender affiliates of RBC, TD, Merrill and Wells under a US$150 million unsecured extendible revolving facility (the “Revolving Facility”). The Revolving Facility is unsecured. AltaGas and its subsidiaries are presently in compliance with the terms of the Revolving Facility and none of the lenders have waived a breach of the agreement governing such Revolving Facility since its execution.

In addition, as at November 5, 2025, AltaGas has an undrawn $70 million demand revolving credit facility with a lender affiliate of RBC (the “Demand Facility”). The Demand Facility is unsecured. AltaGas and its subsidiaries are presently in compliance with the terms of the Demand Facility and the lender has not waived a breach of the agreement governing the Demand Facility since its execution.

The decision to distribute the Common Shares offered hereunder and the determination of the terms of the Offering were made through arm's length negotiations between the Corporation and the Joint Bookrunners, on behalf of the Underwriters. The lenders referenced above did not have any involvement in such decision or determination. The Underwriters will not receive any benefit in connection with the Offering other than the applicable Underwriters’ Fee payable to the Underwriters pursuant to the Underwriting Agreement and AltaGas’ lenders may receive a portion of the net proceeds of the Offering from AltaGas as a repayment of outstanding indebtedness under the credit facilities.

Restrictions on Further Issues and Sales

Pursuant to the Underwriting Agreement, the Corporation has agreed not to offer, issue, authorize the offering or issuance of, or agree to offer, sell, issue, grant any option, right or warrant for sale of any Common Shares or financial instruments or securities convertible or exchangeable for Common Shares or announce any intention to do so, in a public offering or by way of a private placement or otherwise, for a period of 90 days following the Closing Date, without the prior written consent of the Underwriters such consent not to be unreasonably withheld or delayed, except in connection with the exchange, transfer, conversion or exercise rights of existing outstanding securities or existing commitments to issue securities or the issuance of stock options or other share-based compensation.

Non-Certificated Inventory System

Other than pursuant to certain exceptions, registration of interests in and transfers of Common Shares held through CDS, or its nominee, will be made electronically through the NCI system of CDS. On Closing, the Corporation, via its transfer agent, will electronically deliver the Offered Shares registered to CDS or its nominee, and no certificates evidencing the Offered Shares will be issued to purchasers of the Offered Shares. AltaGas expects that purchasers of Offered Shares will receive only a customer confirmation or statement from the Underwriter or other registered dealer which is a CDS participant from or through which an Offered Share is purchased in accordance with the practices and procedures of such Underwriter or other registered dealer. Such practices may vary between registered dealers, but generally customer confirmations are issued promptly following execution of a customer order.

Common Shares held in CDS must be purchased, sold and transferred through a CDS participant, which includes securities brokers and dealers, banks and trust companies. All rights of shareholders who hold Common Shares in CDS must be exercised through, and all payments or other property to which such shareholders may be entitled will be made or delivered by CDS or the CDS participant through which the shareholder holds such Common Shares. A holder of an Offered Share participating in the NCI system will not be entitled to a certificate or other instrument from the Corporation or the Corporation’s transfer agent evidencing that person’s interest in or ownership of Offered Shares, nor, to the extent applicable, will such shareholder be shown on the records maintained by CDS, except through an agent who is a CDS participant.

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PRIOR SALES

The following table sets forth the details regarding all issuances of Common Shares or securities that are convertible or exchangeable into Common Shares in the 12-month period preceding the date of this Prospectus Supplement:

Date(s)	Security Issued	Type of Transaction	Number of Common Shares Issued(1)	Price per Common Share(2)
October 1, 2024 – December 31, 2024	Common Shares	Exercise(s) of Options	136,553	$17.42
January 1, 2025 – March 31, 2025	Common Shares	Exercise(s) of Options	946,400	$19.46
April 1, 2025 – June 30, 2025	Common Shares	Exercise(s) of Options	308,781	$19.16
July 1, 2025 – September 30, 2025	Common Shares	Exercise(s) of Options	305,968	$19.24

Notes:
(1) “Common Shares Issued” represents the aggregate number of Common Shares issued pursuant to exercised Options during the period.
(2) Represents the weighted average exercise price for the subject Options exercised during such period.

PRICE RANGE AND TRADING VOLUME OF THE COMMON SHARES

The Common Shares are listed on the TSX under the symbol “ALA”. The following table sets forth the market price ranges and trading volumes for the Common Shares on the TSX, as reported by the TSX, for the 12-month period before the date of this Prospectus Supplement.

Period	High	Low	Volume
	$	$
November 2024	35.39	32.85	16,856,432
December 2024	35.20	32.02	26,112,375
January 2025	35.13	33.07	16,110,760
February 2025	35.50	32.14	13,197,862
March 2025	39.87	35.18	27,368,963
April 2025	40.98	35.33	17,951,188
May 2025	40.91	37.08	18,926,780
June 2025	39.56	37.73	12,104,006
July 2025	41.37	38.14	13,469,797
August 2025	42.31	40.19	12,474,272
September 2025	43.69	40.46	21,446,976
October 2025	43.81	40.25	16,430,173
November 1 - 4, 2025	41.23	39.45	2,359,782

DESCRIPTION OF COMMON SHARES

The Corporation is authorized to issue an unlimited number of Common Shares. As at the close of business on November 4, 2025, the Corporation had 299,486,164 Common Shares outstanding and 963,864 options to purchase Common Shares (“Options”) outstanding. The Options are held by employees of AltaGas, of which 963,864 were exercisable as of such date. The Options have exercise prices ranging from $18.72 to $19.57 and expire from January 6, 2026 to January 4, 2027. For a summary of certain material attributes and characteristics of the Common Shares and the share capital of AltaGas, see “Description of Securities – Common Shares” in the Prospectus and “Capital Structure – Description of Capital Structure” in the AIF, which is incorporated by reference herein.

DIVIDENDS

Dividends are payable by the Company on the Common Shares at the discretion of the Board of Directors of AltaGas. AltaGas has historically paid quarterly cash dividends on its Common Shares on a quarterly basis on or about the last day of each quarter. The first dividend purchasers of Offered Shares are expected to receive will be payable on or about December 31, 2025 to shareholders of record on December 16, 2025.

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ELIGIBILITY FOR INVESTMENT

In the opinion of Torys LLP, counsel to the Corporation, and Blake, Cassels & Graydon LLP, counsel to the Underwriters, based on the current provisions of the Income Tax Act (Canada) and the regulations thereunder (the "Tax Act"), as of the date hereof, provided that the Offered Shares are listed on a "designated stock exchange" (as defined in the Tax Act, which currently includes the TSX) at the time of acquisition, the Offered Shares will be at such time a qualified investment for a trust governed by a registered retirement savings plan ("RRSP"), registered education savings plan ("RESP"), registered retirement income fund ("RRIF"), deferred profit sharing plan ("DPSP"), registered disability savings plan ("RDSP"), tax-free savings account ("TFSA") or first home savings account ("FHSA") (each as defined in the Tax Act and collectively, "Exempt Plans").

Notwithstanding the foregoing, if the Offered Shares are a "prohibited investment" (as defined in the Tax Act) for a trust governed by a TFSA, RRSP, RRIF, RESP, RDSP or FHSA, the holder, annuitant or subscriber thereof, as applicable, will be subject to a penalty tax as set out in the Tax Act. The Offered Shares will not be a prohibited investment for a TFSA, RRSP, RRIF, RESP, RDSP or FHSA provided the holder of the TFSA, RDSP, or FHSA, the annuitant of the RRSP or RRIF or the subscriber of the RESP, as the case may be, (i) deals at arm's length with the Corporation, for purposes of the Tax Act, and (ii) does not have a "significant interest" (as defined in subsection 207.01(4) of the Tax Act) in the Corporation. In addition, the Offered Shares will not be a "prohibited investment" if the Offered Shares are "excluded property" as defined in the Tax Act for trusts governed by a TFSA, RRSP, RRIF, RESP, RDSP or FHSA.

Prospective purchasers who intend to hold their Offered Shares in their Exempt Plan should consult their own tax advisors regarding their particular circumstances.

RISK FACTORS

An investment in the Offered Shares is subject to various risks, including those set out in this Prospectus Supplement, those set out in, or incorporated by reference into, the Prospectus and those inherent in the industries in which AltaGas operates. Before deciding whether to invest in any Offered Shares, potential investors should consider carefully the risks set out herein, as well as those set out in, or incorporated by reference into, the Prospectus. Prospective investors should also consider the categories of risks identified and discussed in the AIF, the Annual MD&A and the Interim MD&A, which are incorporated by reference into the Prospectus.

Risks Relating to the Offering

Use of Net Proceeds

The Corporation expects to use the available net proceeds of the Offering to repay existing indebtedness and to fund future growth projects, as described under the heading "Use of Proceeds", however management and the Board of Directors of the Corporation will have broad discretion in the actual application of the net proceeds. To the extent the net proceeds of the Offering are not used to repay existing indebtedness and to fund future growth projects, such proceeds may remain undeployed in a dilutive manner. In addition, the results and effectiveness of the application of the net proceeds of the Offering are uncertain and if the net proceeds of the Offering are not applied effectively, the Corporation's financial condition and results of operations may be negatively impacted.

Volatility of Market Price of Common Shares

The market price for the Common Shares may be volatile and subject to wide fluctuations in response to numerous factors, many of which are beyond the Corporation's control. As well, certain institutional investors may base their investment decisions on consideration of the Corporation's performance against such institutions' respective guidelines and criteria, and failure to meet such criteria may result in a limited or no investment in the Common Shares

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by those institutions, which could adversely affect the trading price of the Common Shares. There can be no assurance that fluctuations in price and volume will not occur.

Future Sales or Issuances of Securities

The Corporation may sell additional Common Shares or other securities in subsequent offerings. The Corporation may also issue additional securities to finance future activities. The Corporation cannot predict the size of future issuances of securities or the effect, if any, that future issuances and sales of securities will have on the market price of the Common Shares. Sales or issuances of substantial numbers of Common Shares, or the perception that such sales could occur, may adversely affect prevailing market prices of the Common Shares. With any additional sale or issuance of Common Shares, investors will suffer dilution to their voting power and the Corporation may experience dilution in its earnings per Common Share.

Positive Return Not Guaranteed

There is no guarantee that the Common Shares will earn any positive return in the short term or long term. A holding of Common Shares is highly speculative and involves a high degree of risk and should be undertaken only by holders whose financial resources are sufficient to enable them to assume such risks and who have no need for immediate liquidity in their investment. A holding of Common Shares is appropriate only for holders who have the capacity to absorb a loss of some or all of their holdings.

Investment Eligibility

There can be no assurance that the Common Shares will continue to be qualified investments for Exempt Plans under the Tax Act. The Tax Act imposes penalties for the acquisition or holding of non-qualified investments by Exempt Plans or prohibited investments by Exempt Plans.

INTERESTS OF EXPERTS

The auditors of AltaGas are Ernst & Young LLP, Chartered Professional Accountants, 2200, 215 2nd Street S.W., Calgary, Alberta T2P 1M4. Ernst & Young LLP has confirmed that they are independent with respect to AltaGas in accordance with the Rules of Professional Conduct of the Chartered Professional Accountants of Alberta.

Certain legal matters in connection with the Offering are being passed upon on behalf of the Corporation by Torys LLP and on behalf of the Underwriters by Blake, Cassels & Graydon LLP. As at the date of this Prospectus Supplement, the partners and associates of Torys LLP and the partners and associates of Blake, Cassels & Graydon LLP, as a group, beneficially own, directly or indirectly, less than 1% of the outstanding Common Shares of AltaGas.

TRANSFER AGENT AND REGISTRAR

The transfer agent and registrar for the Common Shares is Computershare Trust Company of Canada at its principal offices in Toronto, Ontario and Calgary, Alberta.

ENFORCEMENT OF JUDGMENTS AGAINST FOREIGN PERSONS OR COMPANIES

Mr. Jon-Al Duplantier and Mr. William (Bill) Bullock are directors of AltaGas and reside outside of Canada. Mr. Duplantier and Mr. Bullock have each appointed Torys LLP as agent for service of process in Canada at 525 – 8th Avenue S.W., 46th Floor, Calgary, Alberta T2P 1G1.

Purchasers are advised that it may not be possible for purchasers to enforce judgments obtained in Canada against any person who resides outside of Canada, even if the party has appointed an agent for service of process.

PURCHASERS' STATUTORY RIGHTS

Securities legislation in certain of the provinces of Canada provides purchasers with the right to withdraw from an agreement to purchase securities. This right may be exercised within two business days after the later of (a) the date

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that the Corporation (i) filed this Prospectus Supplement or any amendment on SEDAR+, and (ii) issued and filed a news release on SEDAR+ announcing that the document is accessible through SEDAR+, and (b) the date that the purchaser or subscriber has entered into an agreement to purchase the securities or a contract to purchase or a subscription for the securities. In several of the provinces, the securities legislation further provides a purchaser with remedies for rescission or, in some jurisdictions, revisions of the price or damages if the prospectus and any amendment contains a misrepresentation or is not delivered to the purchaser, provided that the remedies for rescission, revision of the price or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser's province. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser's province for the particulars of these rights or consult with a legal adviser.

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CERTIFICATE OF THE UNDERWRITERS

Dated: November 5, 2025

To the best of our knowledge, information and belief, the short form prospectus, together with the documents incorporated in the prospectus by reference, as supplemented by the foregoing, constitutes full, true and plain disclosure of all material facts relating to the securities offered by the prospectus and this supplement as required by the securities legislation of each of the provinces of Canada.

CIBC WORLD MARKETS INC.
By: (signed) “Douglas Pearce”
RBC DOMINION SECURITIES INC.
By: (signed) “Curtis Dunford”
BMO NESBITT BURNS INC.
By: (signed) “Tim Lisevich”
MERRILL LYNCH CANADA INC.
By: (signed) “Jamie Hancock”
MIZUHO SECURITIES CANADA INC.
By: (signed) “James Watts”
PETERS & CO. LIMITED
By: (signed) “Cameron Plewes”
MORGAN STANLEY CANADA LIMITED
By: (signed) “Shelly Saidova”

TD SECURITIES INC.
By: (signed) “Scott Barron”
SCOTIA CAPITAL INC.
By: (signed) “Dan Beck”
NATIONAL BANK FINANCIAL INC.
By: (signed) “Tuc Tuncay”
WELLS FARGO SECURITIES CANADA, LTD.
By: (signed) “Darin Deschamps”
ATB SECURITIES INC.
By: (signed) “Robin Hemminger”
J.P. MORGAN SECURITIES CANADA INC.
By: (signed) “Sam Johnson”
TUDOR, PICKERING, HOLT & CO. SECURITIES – CANADA, ULC
By: (signed) “Derek Wheatley”

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