Sec Form 3 Filing - Gallagher Thomas P. @ MIAMI INTERNATIONAL HOLDINGS, INC. - 2025-08-13

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Gallagher Thomas P. 2. Issuer Name and Ticker or Trading Symbol MIAMI INTERNATIONAL HOLDINGS, INC. [ MIAX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chairman & CEO (Last) (First) (Middle) C/O MIAMI INTERNATIONAL HOLDINGS, INC, 7 ROSZEL ROAD, SUITE 1A 3. Date of Earliest Transaction (MM/DD/YY) 08/13/2025 (Street) PRINCETON, NJ08540 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 1,904,599 I By Gallagher Investments, LLC ( 1 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series B Preferred Stock ( 2 ) ( 2 ) ( 2 ) Common Stock 500 I By Gallagher Investments, LLC ( 1 ) Nonvoting Common Stock ( 3 ) ( 3 ) ( 3 ) Common Stock 573,963 D Nonvoting Common Stock ( 3 ) ( 3 ) ( 3 ) Common Stock 135,395 I By Gallagher Investments, LLC ( 1 ) Incentive Stock Option (Right to Buy) $ 12 ( 4 ) 08/02/2026 Nonvoting Common Stock 24,999 D Incentive Stock Option (Right to Buy) $ 12 ( 4 ) 05/28/2028 Nonvoting Common Stock 16,666 D Incentive Stock Option (Right to Buy) $ 12 ( 4 ) 07/30/2029 Nonvoting Common Stock 8,333 D Incentive Stock Option (Right to Buy) $ 15.22 ( 4 ) 01/27/2031 Nonvoting Common Stock 13,140 D Incentive Stock Option (Right to Buy) $ 22.4 ( 5 ) 06/15/2035 Common Stock 13,392 D Nonqualified Stock Option (Right to Buy) $ 12 ( 6 ) 08/02/2026 Nonvoting Common Stock 350,001 I By Gallagher Investments, LLC ( 1 ) Nonqualified Stock Option (Right to Buy) $ 12 ( 6 ) 05/28/2028 Nonvoting Common Stock 283,334 I By Gallagher Investments, LLC ( 1 ) Nonqualified Stock Option (Right to Buy) $ 12 ( 6 ) 07/30/2029 Nonvoting Common Stock 241,667 I By Gallagher Investments, LLC ( 1 ) Nonqualified Stock Option (Right to Buy) $ 15.22 ( 6 ) 01/27/2031 Nonvoting Common Stock 286,860 I By Gallagher Investments, LLC ( 1 ) Nonqualified Stock Option (Right to Buy) $ 16.14 ( 6 ) 09/09/2031 Nonvoting Common Stock 375,000 I By Gallagher Investments, LLC ( 1 ) Nonqualified Stock Option (Right to Buy) $ 22.4 ( 7 ) 06/15/2035 Common Stock 97,858 I By Gallagher Investments, LLC ( 1 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Gallagher Thomas P. C/O MIAMI INTERNATIONAL HOLDINGS, INC 7 ROSZEL ROAD, SUITE 1A PRINCETON, NJ08540 X Chairman & CEO Signatures /s/Alessandra Maria Corona Henriques, Attorney-in-Fact 08/13/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Mr. Gallagher maintains beneficial ownership, including dispositive and voting control, over Gallagher Investments, LLC. ( 2 )The shares of Series B Preferred Stock are convertible on a one-for-one basis into shares of common stock commencing on the Company's initial public offering of its common stock ("IPO") effective date, which is August 13, 2025. The Series B Preferred Stock has no expiration date. ( 3 )The shares of nonvoting common stock are convertible on a one-for-one basis into shares of common stock commencing at any time. The nonvoting common stock has no expiration date. ( 4 )The incentive stock options are fully vested. The options were granted initially as the right to buy nonvoting common stock. On the closing date of the IPO, these will convert to the right to buy common stock instead of nonvoting common stock. ( 5 )4,464 of the shares subject to this incentive stock option will vest on each of June 16, 2026, June 16, 2027 and June 16, 2028, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 6 )The nonqualified stock options are fully vested. The options were granted initially as the right to buy nonvoting common stock. On the closing date of the IPO, these will convert to the right to buy common stock instead of nonvoting common stock. ( 7 )32,620 of the shares subject to this nonqualified stock option will vest on June 16, 2026, 32,619 will vest on June 16, 2027 and the remaining 32,619 will vest on June 16, 2028, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date.

Remarks: See Exhibit 24.1 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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