Exhibit 5.1

Harney Westwood & Riegels

14th Floor, Alexandra House

18 Chater Road

Central

Hong Kong

Tel: +852 5806 7800

Fax: +852 5806 7810

21 May 2026

067935.0001

Alpex Acquisition Corporation

4th Floor, Harbour Place

103 South Church Street

P.O. Box 10240

Grand Cayman KY1-1002

Cayman Islands

Dear Sir or Madam

Alpex Acquisition Corporation (Company)

We are lawyers qualified to practise in the Cayman Islands and have acted as Cayman Islands legal advisers to the Company in connection with the Company’s registration statement on Form S-1 (Registration Statement) to be filed with the Securities and Exchange Commission (Commission), relating to the registration of:

10,000,000 units (Units), each consisting of:

one Class A Ordinary Share (as defined below);

one redeemable warrant, each warrant exercisable to purchase one Class A Ordinary Share at a price of $11.50 per share (Warrants); and

one right to receive one-fourth (1/4) of a Class A Ordinary Share (Rights),

up to 1,500,000 Units, which may be issued upon exercise of an option granted to the underwriters to cover over-allotments, if any, exercisable for a period of 45 days after the closing of the offering (Over-Allotment Units);

all Class A Ordinary Shares, all Warrants and all Rights issued as part of the Units and the Over-Allotment Units;

The British Virgin Islands is Harneys Hong Kong office’s main jurisdiction of practice.

Jersey legal services are provided through a referral arrangement with Harneys (Jersey) which is an independently owned and controlled Jersey law firm.

WPT Lee | IN Mann | BP McCosker | R Ng | PJ Sephton

Anguilla | Bermuda | British Virgin Islands

Cayman Islands | Cyprus | Dubai | Hong Kong | Jersey

London | Luxembourg | Shanghai | Singapore

harneys.com

all Class A Ordinary Shares that may be issued upon the consummation of an initial business combination in respect of the Warrants and the Rights included in the Units and the Over-Allotment Units; and

200,000 Class A Ordinary Shares (or up to 230,000 Class A Ordinary Shares if the Over-Allotment Units is exercised in full) (the Representative Shares) to D. Boral Capital LLC, the representative of the underwriters, or its designees,

in each case under the United States Securities Act of 1933, as amended (Securities Act) and pursuant to the terms of the Registration Statement. In this opinion Companies Act means the Companies Act, as amended, of the Cayman Islands.

Each four (4) Rights entitle the holder thereof to receive one (1) Class A Ordinary Share upon the consummation of a business combination. There will be no fractional shares issued upon the conversion of the Rights.

The Class A Ordinary Shares underlying the Units and the Over-Allotment Units are referred to herein as the Unit Shares, the Class A Ordinary Shares to be issued in accordance with the Warrants are referred to herein as the Warrant Shares, and the Class A Ordinary Shares to be issued in accordance with the Rights are referred to herein as the Rights Shares.

We are furnishing this opinion as Exhibit 5.1 to the Registration Statement.

For the purposes of giving this opinion, we have examined the Documents (as defined in Schedule 1) which are all the documents which we consider necessary and appropriate for the matters set out in this legal opinion. We have not examined any other documents, official or corporate records or external or internal registers and have not undertaken or been instructed to undertake any further enquiry or due diligence in relation to the transaction which is the subject of this opinion.

In giving this opinion, we have relied upon the assumptions set out in Schedule 2 which we have not independently verified.

Based solely upon the foregoing examinations and assumptions and upon such searches as we have conducted and having regard to legal considerations which we deem relevant, and subject to the qualifications set out in Schedule 3, we are of the opinion that under the laws of the Cayman Islands:

Existence and Good Standing. The Company has been duly incorporated as an exempted company with limited liability and is validly existing and in good standing under the laws of the Cayman Islands. It is a separate legal entity and is subject to suit in its own name.

Capacity and Power. The delivery of the Registration Statement by the Company and the performance of its obligations thereunder are within the corporate capacity and power of the Company and have been duly authorised and approved by all necessary corporate action of the Company. The execution, delivery and performance of the Unit Certificate, the Warrant Documents and the Rights Documents (each as defined in Schedule 1) have been authorised by and on behalf of the Company and, once the Unit Certificate, the Warrant Documents and the Rights Documents have been executed and delivered by any director or officer of the Company in accordance with the Resolutions (as defined in Schedule 1), the Unit Certificate, the Warrant Documents and the Rights Documents will be duly executed and delivered on behalf of the Company and will constitute the legal, valid and binding obligations of the Company enforceable in accordance with their terms.

Authorised Share Capital. Based on our review of the Restated M&A (as defined in Schedule 1), the authorised share capital of the Company, with effect from the date that the Restated M&A becomes effective, will be US$50,000 divided into 500,000,000 shares of a par value of US$0.0001 each, comprising (i) 400,000,000 class A ordinary shares of a par value of US$0.0001 each (the Class A Ordinary Shares), (ii) 90,000,000 class B ordinary shares of a par value of US$0.0001 each and (iii) 10,000,000 preferred shares of a par value of US$0.0001 each.

Unit Shares. The Unit Shares to be issued by the Company as contemplated by the Registration Statement have been duly authorised and, when allotted, issued and fully paid for in accordance with the Registration Statement, the Resolutions, and the terms of the Unit Certificate, and when the name of the shareholder is entered in the register of members of the Company (Register of Members), the Unit Shares will be validly issued, allotted, fully paid, and non-assessable and there will be no further obligation on the holder of any of the Unit Shares to make any further payment to the Company in respect of such Unit Shares.

	5	Warrant Shares. The Warrant Shares to be issued by the Company as contemplated by the Registration Statement have been duly authorised and, when allotted, issued and fully paid for in accordance with the Registration Statement, the Resolutions, and the terms of the Warrants Documents, and when the name of the shareholder is entered in the Register of Members, the Warrant Shares will be validly issued, allotted, fully paid, and non-assessable and there will be no further obligation on the holder of any of the Warrant Shares to make any further payment to the Company in respect of such Warrant Shares.

	6	Rights Shares. The Rights Shares to be issued by the Company as contemplated by the Registration Statement have been duly authorised and, when allotted, issued and fully paid for in accordance with the Registration Statement, the Resolutions, and the terms of the Rights Documents, and when the name of the shareholder is entered in the Register of Members, the Rights Shares will be validly issued, allotted, fully paid, and non-assessable and there will be no further obligation on the holder of any of the Rights Shares to make any further payment to the Company in respect of such Rights Shares.

Representative Shares. The Representative Shares to be issued by the Company as contemplated by the Registration Statement have been duly authorised and, when allotted, issued and fully paid for in accordance with the Registration Statement and the Resolutions, and when the name of the shareholder is entered in the Register of Members, the Representative Shares will be validly issued, allotted, fully paid, and non-assessable and there will be no further obligation on the holder of any of the Representative Shares to make any further payment to the Company in respect of such Representative Shares.

Cayman Islands Law. The statements under the caption “Enforceability of Civil Liability”, “Cayman Islands Tax Considerations” and “Description of Securities” in the prospectus forming part of the Registration Statement, to the extent that they constitute statements of Cayman Islands law, are accurate in all material respects as at the date of this opinion and such statements constitute our opinion.

This opinion is confined to the matters expressly opined on herein and given on the basis of the laws of the Cayman Islands as they are in force and applied by the Cayman Islands courts at the date of this opinion. We have made no investigation of, and express no opinion on, the laws of any other jurisdiction. Except as specifically stated herein, we express no opinion as to matters of fact.

In connection with the above opinion, we hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to our name under the heading “Legal Matters” and elsewhere in the prospectus included in the Registration Statement. In giving such consent, we do not thereby admit that we come within the category of persons whose consent is required under Section 7 of the Securities Act, as amended, or the Rules and Regulations of the Commission thereunder.

This opinion is limited to the matters referred to herein and shall not be construed as extending to any other matter or document not referred to herein.

This opinion shall be construed in accordance with the laws of the Cayman Islands.

Yours faithfully

/s/ Harney Westwood & Riegels

Harney Westwood & Riegels

SCHEDULE 1

List of Documents and Records Examined

A copy of the certificate of incorporation of the Company dated 5 January 2026.

A copy of the memorandum and articles of association of the Company dated 5 January 2026 (the M&A);

A copy of the amended and restated memorandum and articles of association of the Company to be adopted by a special resolution and effective upon the listing of the Company’s class A ordinary shares of a par value of US$0.0001 each on the Designated Stock Exchange (as defined therein) in the form as attached to the Registration Statement (the Restated M&A).

A certificate of good standing dated 20 May 2026 in respect of the Company issued by the Registrar of Companies in the Cayman Islands.

A copy of the register of members provided to us provided to us on 4 May 2026.

A copy of the register of directors and officers of the Company provided to us on 4 May 2026.

Copies of the unanimous written resolutions of the directors of the Company dated 1 April 2026 and 21 May 2026, respectively (Resolutions).

A certificate dated 21 May 2026 provided by each director of the Company confirming certain matters to us which are relevant to our opinion (Director’s Certificate), a copy of which is annexed to this opinion as Appendix I.

Items 1 to 8 above are together referred to as the Corporate Documents.

The Registration Statement.

A draft of the form of the unit certificate representing the Units (the Unit Certificate).

A draft of the form of the warrant agreement and the warrant certificate constituting the Warrants (the Warrant Documents).

A draft of the form of the rights agreement and the rights certificate constituting the Rights (the Rights Documents).

1 to 12 above are collectively referred to in this opinion as the Documents.

Schedule 2

Assumptions

Validity under Foreign Laws. That:

(a)

all formalities required under any applicable laws (other than the laws of the Cayman Islands) have been complied with; and

(b)

no other matters arising under any foreign law will affect the views expressed in this opinion.

Authenticity of Documents. All original Documents are authentic, all signatures, initials and seals are genuine, all copies of Documents are true and correct copies and the Registration Statement conforms in every material respect to the latest drafts of the same produced to us and, where the Registration Statement has been provided to us in successive drafts marked-up to indicate changes to such documents, all such changes have been so indicated.

Corporate Documents. All matters required by law to be recorded in the Corporate Documents are so recorded, and all corporate minutes, resolutions, certificates, documents and records which we have reviewed are accurate and complete, and all facts expressed in or implied thereby are accurate and complete.

Director’s Certificate. The contents of the Director’s Certificate are true and accurate as at the date of this opinion.

Constitutional Documents. The M&A is the latest memorandum and articles of association of the Company in effect as of the time of the opinion.

Resolutions. The Resolutions have been duly executed (and whereby a corporate entity such execution has been duly authorised if so required) by or on behalf of the directors, or by or on behalf of each shareholder in respect of the shareholder resolutions, and the signatures and initials thereon are those of a person or persons in whose name the Resolutions have been expressed to be signed. The Resolutions remain in full force and effect.

No Steps to Wind-up. The directors and shareholders of the Company have not taken any steps to have the Company struck off or placed in liquidation, no steps have been taken to wind up the Company and no receiver has been appointed over any of the property or assets of the Company.

Unseen Documents. Save for the Documents provided to us there are no resolutions, agreements, documents or arrangements which materially affect, amend or vary the transactions envisaged in the Documents and, in particular, that the entry into and performance of the transactions contemplated under the Registration Statement will not cause any of the parties thereto to be in breach of any agreement or undertaking.

Schedule 3

Qualifications

Non-assessable. The term non-assessable means, with respect to shares in the Company, that a shareholder shall not, solely by virtue of its status as a shareholder, be liable for additional assessments or calls on the shares by the Company or its creditors (except in exceptional circumstances, such as involving fraud, the establishment of an agency relationship or an illegal or improper purpose or other circumstances in which a court may be prepared to pierce or lift the corporate veil).

Foreign Statutes. We express no opinion in relation to provisions making reference to foreign statutes in the Registration Statement.

Commercial Terms. Except as specifically stated herein, we make no comment with respect to any representations and warranties which may be made by or with respect to the Company in any of the documents or instruments cited in this opinion or otherwise with respect to the commercial terms of the transactions the subject of this opinion.

Register of members. Under the Companies Act, the register of members of a Cayman Islands company is by statute regarded as prima facie evidence of any matters which the Companies Act directs or authorises to be inserted therein. A third party interest in the shares in question would not appear. An entry in the register of members may yield to a court order for rectification (for example, in the event of fraud or manifest error).

Good Standing. The Company shall be deemed to be in good standing at any time if all fees (including annual filing fees) and penalties under the Companies Act have been paid and the Registrar of Companies in the Cayman Islands has no knowledge that the Company is in default under the Companies Act.

Economic Substance. We have undertaken no enquiry and express no view as to the compliance of the Company with the International Tax Co-operation (Economic Substance) Act (Revised).

Appendix I

Director’s Certificate