As filed with the U.S. Securities and Exchange Commission on January 5, 2026

Registration No. 333-290602

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

AMENDMENT NO. 4
TO

FORM S-1

REGISTRATION STATEMENT UNDER

THE SECURITIES ACT OF 1933

Black Spade Acquisition III Co

(Exact Name of Registrant as Specified in Its Charter)

Suite 2902, 29/F, The Centrium, 60 Wyndham Street,
Central, Hong Kong

Tel: +852 3955 1316

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Cogency Global Inc.

122 East 42nd Street, 18th Floor

New York, NY 10168

Phone: (800) 221-0102

Fax: (800) 944-6607

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this registration statement. If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933 check the following box. ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ¨

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until the Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

EXPLANATORY NOTE

This Amendment No. 4 (the “Amendment No. 4”) to the Registration Statement on Form S-1 (File No. 333-290602) of Black Spade Acquisition III Co (the “Registration Statement”) is being filed as an exhibits-only filing. Accordingly, this Amendment No. 4 consists only of the facing page, this explanatory note, Part II of the Registration Statement, the signature page to the Registration Statement and the filed exhibit. The remainder of the Registration Statement is unchanged and has therefore been omitted.

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

The estimated expenses payable by us in connection with the offering described in this registration statement (other than the underwriting discount and commissions) will be as follows:

Cayman Islands law does not limit the extent to which a company’s memorandum and articles of association may provide for indemnification of officers and directors, except to the extent any such provision may be held by the Cayman Islands courts to be contrary to public policy, such as to provide indemnification against willful default, willful neglect, civil fraud or the consequences of committing a crime. Our amended and restated memorandum and articles of association provide for indemnification of our officers and directors to the maximum extent permitted by law, including for any liability incurred in their capacities as such, except through their own actual fraud, willful default or willful neglect. We may purchase a policy of directors’ and officers’ liability insurance that insures our officers and directors against the cost of defense, settlement or payment of a judgment in some circumstances and insures us against our obligations to indemnify our officers and directors.

Our officers and directors have agreed to waive any right, title, interest or claim of any kind in or to any monies in the trust account, and have agreed to waive any right, title, interest or claim of any kind they may have in the future as a result of, or arising out of, any services provided to us and will not seek recourse against the trust account for any reason whatsoever. Accordingly, any indemnification provided will only be able to be satisfied by us if (i) we have sufficient funds outside of the trust account or (ii) we consummate an initial business combination.

Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers or persons controlling us pursuant to the foregoing provisions, we have been informed that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act and is therefore unenforceable.

On September 5, 2025, our initial shareholders paid $25,000, or approximately $0.004 per share, in exchange for 5,750,000 Class B ordinary shares (the “founder shares”). Such securities were issued in connection with our organization pursuant to the exemption from registration contained in Section 4(a)(2) of the Securities Act. The number of founder shares outstanding was determined based on the expectation that the total size of this offering would be a maximum of 17,250,000 units if the underwriters’ over- allotment option is exercised in full and therefore that such founder shares would represent 25% of the outstanding shares after this offering. Up to 750,000 of these shares will be surrendered for no consideration depending on the extent to which the underwriters’ over-allotment is exercised (including the forfeiture of 82,174 shares out of 630,000 founder shares held by our directors, officers, advisors and employees of sponsor’s affiliates).

Black Spade Sponsor LLC III, our sponsor, is an accredited investor for purposes of Rule 501 of Regulation D. Each of the equity holders in our sponsor is an accredited investor under Rule 501 of Regulation D. The sole business of our sponsor is to act as the company’s sponsor in connection with this offering. The limited liability company agreement of our sponsor provides that its membership interests may only be transferred to our officers or directors or other persons affiliated with our sponsor, or in connection with estate planning transfers.

Our sponsor and the underwriters have committed to purchase an aggregate of 7,550,000 private placement warrants (or 8,150,000 private placement warrants if the underwriters’ over-allotment option is exercised in full), each exercisable to purchase one Class A ordinary share at $11.50 per share, at a price of $0.50 per warrant, or $3,775,000 in the aggregate (or $4,075,000 if the underwriters’ over-allotment option is exercised in full), in a private placement that will close simultaneously with the closing of this offering. Of those 7,550,000 private placement warrants (or 8,150,000 private placement warrants if the underwriters’ over-allotment option is exercised in full), our sponsor has agreed to purchase 6,550,000 private placement warrants (or 7,000,000 private placement warrants if the underwriters’ over-allotment option is exercised in full), and the underwriters have agreed to purchase 1,000,000 private placement warrants (or 1,150,000 private placement warrants if the underwriters’ over-allotment option is exercised in full). This purchase will take place on a private placement basis simultaneously with the completion of our initial public offering. This issuance will be made pursuant to the exemption from registration contained in Section 4(a)(2) of the Securities Act.

No underwriting discounts or commissions will be paid with respect to such sales.

EXHIBIT INDEX

# Filed herewith.

* Previously filed.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the Hong Kong Special Administrative Region of the People’s Republic of China, on the 5th day of January, 2026.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

AUTHORIZED REPRESENTATIVE

Pursuant to the requirements of Section 6(a) of the Securities Act of 1933, the undersigned has signed this registration statement, solely in its capacity as the duly authorized representative of Black Spade Acquisition III Co, in the City of New York, New York, on the 5th day of January, 2026.