[SCHEDULE 13D] Miami International Holdings, Inc. SEC Filing

Rhea-AI Filing Summary

Thomas P. Gallagher and Gallagher Investments LLC reported beneficial ownership of 4,226,457 shares (5.1% of outstanding common stock) of Miami International Holdings, Inc. The filing shows Gallagher holds 573,963 shares directly, 12,000 jointly with his spouse, and options exercisable for 63,138 shares; Gallagher Investments holds 2,040,494 shares and options exercisable for 1,536,862 shares. Many shares resulted from conversions tied to the issuer's initial public offering and a small purchase of 12,000 IPO shares on August 14, 2025. Holdings are declared for investment with a 180-day lock-up and Gallagher serves as the issuer's Chairman and CEO, retaining voting and dispositive control over the shares reported.

Insights

Financial Analyst

TL;DR: Founder/CEO holds a meaningful but non-control stake (5.1%), combining direct shares and exercisable options after the IPO.

The Schedule 13D documents a 5.1% aggregate position based on 80,947,066 shares outstanding, including significant option positions exercisable within 60 days. Conversions from Series B Preferred and nonvoting common occurred in connection with the IPO, concentrating equity with management. The 180-day lock-up reduces near-term supply of shares from insiders, which can support post-IPO price stability. For models, treat exercisable options as potential dilution but acknowledge they are currently in an insider position and exercised under documented conversions and transfers.

Corporate Governance Expert

TL;DR: As Chairman and CEO, Gallagher's combined direct and indirect holdings give him meaningful influence, and standard lock-up provisions apply.

The filing confirms Gallagher's sole voting and dispositive power over his direct holdings and managerial control of Gallagher Investments, which holds additional shares and exercisable options. This dual structure centralizes control in management hands, potentially affecting shareholder oversight and corporate decision-making. The absence of other arrangements or litigation disclosures is noted, and the 180-day underwriter lock-up is standard for IPO participants. Monitor future amendments or dispositions for governance implications.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

Miami International Holdings, Inc. (the "Issuer")

(Name of Issuer)

Common Stock, $0.001 par value per share (the "Common Stock")

(Title of Class of Securities)

59356Q108

(CUSIP Number)

Thomas P. Gallagher
c/o Miami International Holdings, Inc., 7 Roszel Road, Suite 1A
Princeton, NJ, 08540
6098977300

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

08/14/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SCHEDULE 13D

CUSIP No. 59356Q108

1

Name of reporting person

Thomas P. Gallagher

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF, SC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 649,101.00
8	Shared Voting Power 3,577,356.00
9	Sole Dispositive Power 649,101.00
10	Shared Dispositive Power 3,577,356.00

11

Aggregate amount beneficially owned by each reporting person

4,226,457.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

5.1 %

14

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13D

CUSIP No. 59356Q108

1

Name of reporting person

Gallagher Investments LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

OO

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 3,577,356.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 3,577,356.00

11

Aggregate amount beneficially owned by each reporting person

3,577,356.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

4.3 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, $0.001 par value per share (the "Common Stock")
(b)	Name of Issuer: Miami International Holdings, Inc. (the "Issuer")
(c)	Address of Issuer's Principal Executive Offices: 7 Roszel Road, Suite 1A, Princeton, NEW JERSEY , 08540.
Item 2.	Identity and Background
(a)	This Schedule 13D is filed by the following persons (collectively, the "Reporting Persons"), pursuant to their agreement to the joint filing of this Schedule 13D attached hereto as Exhibit 1: (i) Thomas P. Gallagher, an individual resident of the State of New Jersey; and (ii) Gallagher Investments LLC, a New Jersey limited liability company
(b)	The address for each of the Reporting Persons is c/o Miami International Holdings, Inc. 7 Roszel Road, Suite 1A, Princeton, New Jersey 08540
(c)	Mr. Gallagher's principal occupation is Chairman and Chief Executive Officer of the Issuer, and the principal business of Gallagher Investments LLC ("Gallagher Investments") is holding the investments of Mr. Gallagher.
(d)	During the past five years neither of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).
(e)	During the past five years neither of the Reporting Persons was a party to a civil proceeding or a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoying future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.
(f)	United States
Item 3.	Source and Amount of Funds or Other Consideration
	Mr. Gallagher acquired the Common Stock beneficially owned by him for cash using personal funds or as compensation for serving as an executive officer, chairman and co-founder of the Issuer. On August 11, 2025, Mr. Gallagher transferred to Gallagher Investments 1,904,599 shares of Common Stock, 500 shares of Series B Preferred Stock, 135,395 shares of Nonvoting Common Stock, respectively, and options to purchase 1,536,862 shares of Nonvoting Common Stock. On August 15, 2025, (i) Gallagher Investments received 135,895 shares of the Issuer's Common Stock upon the conversion of 500 shares of Series B Preferred Stock and 135,395 shares of Nonvoting Common Stock into shares of Common Stock in connection with the Issuer's initial public offering without consideration, and (ii) Mr. Gallagher received 573,963 shares of Common Stock upon the conversion of the Issuer's Nonvoting Common Stock into shares of Common Stock in connection with the Issuer's initial public offering, in each case without consideration. On August 14, 2025, the trade date, Mr. Gallagher purchased 12,000 shares of Common Stock in the Issuer's initial public offering for cash using personal funds.
Item 4.	Purpose of Transaction
	The Reporting Persons acquired all of their securities for investment purposes only and are being held as a long-term investment. The Reporting Persons intend to continuously review their investment in the Issuer and may in the future determine to acquire additional securities of the Issuer or dispose of the securities of the Issuer owned by them or take any other available course of action, including surrendering or selling shares back to the Issuer for tax withholding obligations. Notwithstanding anything contained herein, the Reporting Persons specifically reserve the right to change their intention with respect to any or all of such matters. Mr. Gallagher is the Chairman and Chief Executive Officer of the Issuer and acquired his securities in that capacity. As a result, Mr. Gallagher may have influence over the corporate activities of the Issuer, including activities that may relate to items described in subparagraphs (a) through (j) of Item 4 of Schedule 13D. The Reporting Persons do not have any present plans or proposals as of the date hereof that relate to or would result in any of the transactions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.
Item 5.	Interest in Securities of the Issuer
(a)	The Reporting Persons beneficially own an aggregate of 4,226,457 shares of Common Stock as follows: (i) 573,963 shares of Common Stock held by Mr. Gallagher, (ii) 12,000 shares of Common Stock held by Mr. Gallagher jointly with his spouse, (iii) stock options exercisable for 63,138 shares of Common Stock held by Mr. Gallagher, which are exercisable within 60 days of the date hereof, (iv) 2,040,494 shares of Common Stock held by Gallagher Investments, and (v) stock options exercisable for 1,536,862 shares of Common Stock held by Gallagher Investments, which are exercisable within 60 days of the date hereof. Mr. Gallagher maintains beneficial ownership, including dispositive and voting control, over Gallagher Investments. The percentage ownership is based on 80,947,066 shares of Common Stock outstanding at the close of business on August 15, 2025.
(b)	Mr. Gallagher has the sole power to vote or to direct the vote, sole power to dispose or to direct the disposition of all shares beneficially owned by him and shares the power to vote or to direct the vote, and shares power to dispose or to direct the disposition of all shares beneficially owned by Gallagher Investments, of which Mr. Gallagher is the Manager.
(c)	Except as described under Item 3 above, the Reporting Person did not engage in any transactions in shares of the Issuer's Common Stock during the past 60 days.
(d)	Not applicable.
(e)	Not applicable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	As the Chairman and Chief Executive Officer of the Issuer, Mr. Gallagher has in the past and may receive future compensation in the form of Common Stock, options or other securities convertible into Common Stock. The Reporting Persons have entered into agreements with the underwriters of the Issuer's initial public offering pursuant to which the Reporting Persons have agreed not to sell, transfer, pledge or otherwise dispose of any of the shares of common stock for a period of 180 days after the date of the Issuer's initial public offering. There currently are no other contracts, arrangements, understandings or relationships (legal or otherwise) between the Reporting Persons and any other person with respect to any securities of the Issuer, including, but not limited to, transfer or voting of any other securities, finder's fees, joint ventures, loan or option arrangements, puts or calls, guarantees of profits, division of profits or loss, or the giving or withholding of proxies. Pursuant to Rule 13d-1(k) promulgated under the Exchange Act, the Reporting Persons have entered into an agreement attached hereto as Exhibit 1, with respect to the joint filing of this Schedule 13D and any amendment or amendments hereto.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 1. Joint Filing Agreement dated August 21, 2025 by the Reporting Persons Exhibit 2. Form of Lock-Up Agreement (incorporated herein by reference to Exhibit A to the Form of Underwriting Agreement, filed as Exhibit 1.1 to the Issuer's Registration Statement on Form S-1/A, filed on August 4, 2025 (File No. 333-288767)

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Thomas P. Gallagher

Signature:	Thomas P. Gallagher
Name/Title:	/s/ Thomas P. Gallagher, Individually
Date:	08/21/2025

Gallagher Investments LLC

Signature:	Gallagher Investments LLC
Name/Title:	/s/ Thomas P. Gallagher, Manager
Date:	08/21/2025

FAQ

How many shares of MIAX does Thomas P. Gallagher beneficially own?

The reporting persons beneficially own 4,226,457 shares, representing 5.1% of MIAX's outstanding common stock based on 80,947,066 shares.

What portion of Gallagher's holdings are exercisable options?

63,138 options held by Mr. Gallagher and 1,536,862 options held by Gallagher Investments are exercisable within 60 days.

Did Gallagher acquire shares in the IPO and through conversions?

Yes. Conversions from Series B Preferred and Nonvoting Common stock occurred in connection with the IPO, and Mr. Gallagher purchased 12,000 IPO shares on August 14, 2025.

Does Gallagher have voting control over the reported shares?

Yes. Mr. Gallagher has sole voting and dispositive power over his directly held shares and managerial control over Gallagher Investments' shares, which are shared voting/dispositive.

Are there any restrictions on selling the reported shares?

Yes. The Reporting Persons agreed to a 180-day lock-up with the underwriters, restricting sale, transfer, or pledge for that period after the IPO.

Were any legal proceedings disclosed against the reporting persons?

No. The filing states that during the past five years neither reporting person was convicted in a criminal proceeding nor was subject to disclosable civil or administrative judgments.