Exhibit 5.1

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June 12, 2026

Ares Acquisition Corporation III
c/o Ares Management LLC
245 Park Avenue, 44th Floor
New York, NY 10167

Ladies and Gentlemen:

Re: Ares Acquisition Corporation III Registration Statement on Form S-1

We are issuing this opinion in our capacity as special United States counsel to Ares Acquisition Corporation III, a Cayman Island exempted company (the “Company”), in connection with the registration under the Securities Act of 1933, as amended (the “Act”), on a Registration Statement on Form S-1 originally filed with the Securities and Exchange Commission (the “Commission”) on June 12, 2026 (the “Registration Statement”) of 30,000,000 units of the Company, including the underwriters’ over-allotment option to purchase up to an additional 4,500,000 units (collectively, the “Units”), with each Unit consisting of one Class A ordinary share, par value $0.0001 per share (the “Class A Ordinary Shares”), of the Company and one-tenth of one redeemable warrant of the Company, each whole warrant entitling the holder to purchase one Class A Ordinary Share (the “Warrants”).

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K promulgated under the Act.

In rendering the opinions stated herein, we have examined and relied upon the following:

(a)the corporate and organizational documents of the Company, including the form of Amended and Restated Memorandum and Articles of Association of the Company, filed as Exhibit 3.1 to the Registration Statement (the “Memorandum and Articles of Association”);

(b)the form of Underwriting Agreement (the “Underwriting Agreement”) proposed to be entered into by and between the Company and J.P. Morgan Securities LLC and Jefferies LLC, as representatives of the several underwriters named therein (the “Underwriters”), relating to the sale by the Company to the Underwriters of the Units, filed as Exhibit 1.1 to the Registration Statement;

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(c)the form of Unit Certificate, filed as Exhibit 4.1 to the Registration Statement;

(d)the form of Warrant Certificate, filed as Exhibit 4.3 to the Registration Statement;

(e)the form of Warrant Agreement proposed to be entered into by and between the Company and Continental Stock Transfer & Trust Company, as warrant agent (the “Warrant Agreement”), filed as Exhibit 4.4 to the Registration Statement;

(f)minutes and records of the corporate proceedings of the Company with respect to the issuance and sale of the Units; and

(g)the Registration Statement.

For purposes of this opinion, we have examined such other documents, records, certificates, resolutions and other instruments deemed necessary as a basis for this opinion, and we have assumed the authenticity of all documents submitted to us as originals, the conformity to the originals of all documents submitted to us as copies and the authenticity of the originals of all documents submitted to us as copies. We have also assumed the legal capacity of all natural persons, the genuineness of the signatures of persons signing all documents in connection with which this opinion is rendered, the authority of such persons signing on behalf of the parties thereto (other than the Company), and the due authorization, execution and delivery of all documents by the parties thereto (other than the Company). As to any facts material to the opinions expressed herein which we have not independently established or verified, we have relied upon statements and representations of officers and other representatives of the Company and others.

We do not express any opinion with respect to the laws of any jurisdiction other than the laws of the State of New York.

Based upon the foregoing and subject to the limitations, qualifications and assumptions stated herein, we are of the opinion that:

1.When the Units are delivered in accordance with the Underwriting Agreement upon payment of the agreed upon consideration therefor, the Units will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms under the laws of the State of New York.

2.When the Units are delivered in accordance with the Underwriting Agreement upon payment of the agreed upon consideration therefor, the Warrants included in such Units will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms under the laws of the State of New York.

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In addition, in rendering the foregoing opinions we have assumed that:

(a)the Company (i) is duly incorporated and is validly existing and in good standing, (ii) has requisite legal status and legal capacity under the laws of the jurisdiction of its organization and (iii) has complied and will comply with all aspects of the laws of the jurisdiction of its organization in connection with the transactions contemplated by, and the performance of its obligations under, the Warrant Agreement;

(b)the Company has the corporate power and authority to execute, deliver and perform all its obligations under the Warrant Agreement and the Units;

(c)neither the execution and delivery by the Company of the Warrant Agreement nor the performance by the Company of its obligations thereunder, including the issuance and sale of the Units: (i) conflicts or will conflict with the Memorandum and Articles of Association, (ii) constitutes or will constitute a violation of, or a default under, any lease, indenture, instrument or other agreement to which the Company or its property is subject, (iii) contravenes or will contravene any order or decree of any governmental authority to which the Company or its property is subject, or (iv) violates or will violate any law, rule or regulation to which the Company or its property is subject (except that we do not make the assumption set forth in this clause (iv) with respect to the laws of the State of New York); and

(d)neither the execution and delivery by the Company of the Warrant Agreement nor the performance by the Company of its obligations thereunder, including the issuance and sale of the Units, requires or will require the consent, approval, licensing or authorization of, or any filing, recording or registration with, any governmental authority under any law, rule or regulation of any jurisdiction.

Our opinions expressed above are subject to the qualifications that we express no opinion as to the applicability of, compliance with, or effect of: (i) any bankruptcy, insolvency, reorganization, fraudulent transfer, fraudulent conveyance, moratorium or other similar law or judicially developed doctrine in this area (such as substantive consolidation or equitable subordination) affecting the enforcement of creditors’ rights generally, (ii) general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law), including but not limited to: (A) principles limiting the availability of specific performance and injunctive relief; (B) principles which limit the availability of a remedy under certain circumstances where another remedy has been elected; (C) principles requiring reasonableness, good faith and fair dealing in the performance and enforcement of an agreement by the party seeking enforcement; (D) principles which may permit a party to cure a material failure to perform its obligations; and (E) principles affording equitable defenses such as waiver, laches and estoppel, (iii) other commonly recognized statutory and judicial constraints on enforceability including statutes of limitations, (iv) public policy considerations which may limit the rights of parties to

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obtain certain remedies, (v) any requirement that a claim with respect to any security denominated in other than U.S. dollars (or a judgment denominated in other than U.S. dollars in respect of such claim) be converted into U.S. dollars at a rate of exchange prevailing on a date determined in accordance with applicable law, (vi) governmental authority to limit, delay or prohibit the making of payments outside of the United States or in a foreign currency or currency unit, and (vii) any laws except the laws of the State of New York. It is possible that terms in a particular contract covered by our opinions expressed above may not prove enforceable for reasons other than those explicitly cited in this opinion should an actual enforcement action be brought, but (subject to all the exceptions, qualifications, exclusions and other limitations contained in this opinion) such unenforceability would not in our opinion prevent the party entitled to enforce that contract from realizing the principal benefits purported to be provided to that party by the terms in that contract which are covered by our opinions expressed above. In addition, none of the opinions or other advice contained in this opinion covers or otherwise addresses any of the following types of provisions (or the enforceability thereof) which may be contained in the Underwriting Agreement or Warrant Agreement: (i) provisions mandating contribution towards judgments or settlements among various parties; (ii) waivers of benefits and rights to the extent they cannot be waived under applicable law; (iii) provisions providing for liquidated damages, late charges and prepayment charges, in each case if deemed to constitute penalties; or (iv) requirements specifying that provisions thereof may only be waived in writing (these provisions may not be valid, binding or enforceable to the extent that an oral agreement or an implied agreement by trade practice or course of conduct has been created modifying any provision of such documents). We advise you that issues addressed by this opinion may be governed in whole or in part by other laws, but we express no opinion as to whether any relevant difference exists between the laws upon which our opinions are based and any other laws which may actually govern.

For purposes of rendering our opinions expressed above, we have assumed that (i) the Registration Statement remains effective during the offer and sale of the Units, (ii) at the time of the issuance, sale and delivery of each Unit (x) there will not have occurred any change in law affecting the validity, legally binding character or enforceability of such Unit and (y) the issuance, sale and delivery of such Unit, the terms of such Unit and compliance by the Company with the terms of such Unit will not violate any applicable law, any agreement or instrument then binding upon the Company or any restriction imposed by any court or governmental body having jurisdiction over the Company.

We hereby consent to the filing of this opinion with the Commission as Exhibit 5.1 to the Registration Statement. We also consent to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission.

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We do not find it necessary for the purposes of this opinion, and accordingly we do not purport to cover herein, the application of the securities or “Blue Sky” laws of the various states to the offering of the Units. This opinion is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. This opinion speaks only as of the date that the Registration Statement becomes effective under the Act, and we assume no obligation to revise or supplement this opinion should the present laws of the State of New York be changed by legislative action, judicial decision or otherwise after the date hereof.

This opinion is furnished to you in connection with the filing of the Registration Statement and is not to be used, circulated, quoted or otherwise relied upon for any other purposes.

This opinion and consent may be incorporated by reference in a subsequent registration statement on Form S-1 filed pursuant to Rule 462(b) under the Act with respect to the registration of additional securities for sale in the offering contemplated by the Registration Statement and shall cover such additional securities, if any, registered on such subsequent registration statement.

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