UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF THE
SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): June 4, 2026

KEYSTONE ACQUISITION CORP.

(Exact name of registrant as specified in its charter)

142 West 57th Street

11th Floor

New York, New York 10019

(Address of principal executive offices, including zip code)

Registrant’s telephone number, including area code: (408) 482-7532

Not Applicable
(Former name or former address, if changed since last report)

Securities registered pursuant to Section 12(b) of the Act:

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☒

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 13(a) of the Exchange Act. ☐

Item 8.01. Other Events.

As previously reported, on June 4, 2026, Keystone Acquisition Corp. (the “Company”) consummated its initial public offering (“IPO”) of 28,750,000 units (the “Units”), including the issuance of 3,750,000 Units as a result of the underwriters’ exercise of their over-allotment option in full. Each Unit consists of one Class A ordinary share of the Company, par value $0.0001 per share (the “Class A Ordinary Shares”), and one-half of one redeemable warrant of the Company (each whole warrant, a “Warrant”), with each Warrant entitling the holder thereof to purchase one Class A Ordinary Share for $11.50 per share, subject to adjustment. The Units were sold at a price of $10.00 per Unit, generating gross proceeds to the Company of $287,500,000.

Also as previously reported, on June 4, 2026, simultaneously with the consummation of the IPO, the Company completed the private sale (the “Private Placement”) of an aggregate of 8,468,750 warrants (the “Private Placement Warrants”) to Keystone International Acquisition Management LLC (the “Sponsor”), Cohen & Company Capital Markets, a division of Cohen & Company Securities, LLC and Clear Street LLC, as representatives of the underwriters (the “Representatives”) at a purchase price of $1.00 per Private Placement Warrant, generating gross proceeds to the Company of $8,468,750. Of the 8,468,750 Private Placement Warrants, the Sponsor purchased 5,593,750 Private Placement Warrants and the Representatives purchased 2,875,000 Private Placement Warrants.

A total of $288,218,750 of the proceeds from the IPO and Private Placement, which amount includes up to $11,500,000 of the underwriters’ deferred commission, was placed in a U.S.-based trust account maintained by Efficiency INC., acting as trustee.

An audited balance sheet as of June 4, 2026 reflecting receipt of the proceeds upon consummation of the IPO and the Private Placement has been issued by the Company and is included as Exhibit 99.1 to this Current Report on Form 8-K.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

EXHIBIT INDEX

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.