Miami International Holdings (NASDAQ: MIAX) investor discloses 12.2% stake and board seat

Rhea-AI Filing Summary

Horizon Kinetics Asset Management LLC filed Amendment No. 3 to Schedule 13D reporting beneficial ownership of 11,162,809 shares of Miami International Holdings, Inc. common stock, representing 12.2% of the outstanding shares as of the close of business on April 27, 2026. Horizon Kinetics holds sole voting and dispositive power over these shares on behalf of its managed accounts, while parent company Horizon Kinetics Holding Corp may be deemed to share beneficial ownership. The filing states the position is held for investment purposes, with the flexibility to buy more or sell shares over time. On April 24, 2026, Eric Sites, a Managing Director of Horizon Kinetics Asset Management, was elected to the issuer’s Board of Directors.

Insights

Governance and ownership structure analyst

Horizon Kinetics reports a 12.2% stake and gains a board seat at Miami International Holdings.

Horizon Kinetics Asset Management LLC and its parent Horizon Kinetics Holding Corp disclose beneficial ownership of 11,162,809 shares of Miami International Holdings, equal to 12.2% of outstanding common stock as of April 27, 2026. Voting and dispositive power reside with Horizon Kinetics Asset Management on behalf of managed accounts.

The filing describes the position as held for investment purposes, but notes Horizon Kinetics may acquire additional shares or sell holdings in the ordinary course through market or private transactions. Beneficial ownership is expressly disclaimed beyond the firms’ pecuniary interests and those of their managed accounts.

A notable development is the election of Eric Sites, a Managing Director of Horizon Kinetics Asset Management, to Miami International Holdings’ Board of Directors on April 24, 2026. This combination of a double‑digit ownership stake and board representation can increase the investor’s influence on strategy and oversight, though any specific strategic intentions are not detailed in the excerpt provided.

Key Figures

Beneficial ownership: 11,162,809 shares Ownership percentage: 12.2% Sole voting power: 11,162,809 shares +3 more

6 metrics

Beneficial ownership 11,162,809 shares Common stock beneficially owned by Horizon Kinetics Asset Management as of April 27, 2026

Ownership percentage 12.2% Percent of Miami International Holdings common stock outstanding represented by Horizon Kinetics’ position

Sole voting power 11,162,809 shares Shares over which Horizon Kinetics Asset Management has sole voting authority

Sole dispositive power 11,162,809 shares Shares over which Horizon Kinetics Asset Management has sole dispositive power

Event date April 24, 2026 Date that triggered filing requirement and election of Eric Sites to the board

Beneficial ownership date April 27, 2026 As-of date for the 11,162,809 shares and 12.2% ownership disclosure

Key Terms

beneficially owned, Sole Voting Power, Sole Dispositive Power, Managed Accounts, +2 more

6 terms

beneficially owned financial

"As of the close of business on April 27, 2026, HKAM beneficially owned 11,162,809 shares of common stock."

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

Sole Voting Power financial

"Number of Shares Beneficially Owned by Each Reporting Person With: | 7 | Sole Voting Power 11,162,809.00"

Sole voting power is the exclusive right to cast votes attached to a shareholder’s stock without needing approval from anyone else. Like holding the only remote control for a TV, it lets that holder decide corporate matters such as board members, mergers, and policy changes, making it important to investors because it concentrates control and can strongly influence a company’s strategy and the value of its shares.

Sole Dispositive Power financial

"Number of Shares Beneficially Owned by Each Reporting Person With: | 9 | Sole Dispositive Power 11,162,809.00"

Sole dispositive power is the exclusive legal authority to decide what happens to a security — for example, whether to sell, transfer, or retain shares — without needing anyone else’s permission. Investors care because it signals who truly controls the economic outcome of an investment: like holding the only key to a safe, the holder can realize gains or losses and may trigger regulatory reporting, insider rules, or influence over corporate ownership.

Managed Accounts financial

"manages funds and accounts ("Managed Accounts") that hold securities of the Issuer."

Managed accounts are collections of investments owned by an individual or institution but run day-to-day by a professional who buys, sells and allocates assets according to an agreed plan. They matter to investors because they provide tailored oversight, active risk control and potential tax efficiency—like hiring a personal chef to manage your diet—while fees and the manager’s skill directly affect returns.

pecuniary interest financial

"such beneficial ownership is expressly disclaimed, except to the extent of its pecuniary interest therein."

Schedule 13D regulatory

"The following constitutes Amendment No. 3 to the filed by the undersigned."

A Schedule 13D is a legal document that investors file with regulators when they buy a large enough stake in a company to potentially influence its management or decisions. It provides details about the investor’s intention, ownership stake, and plans, helping other investors understand who is gaining control and what their motives might be.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Accepted by SEC EDGAR 04/28/2026 - 03:14 PM Learn about SEC filing dates

FAQ

What stake does Horizon Kinetics hold in Miami International Holdings (MIAX)?

Horizon Kinetics Asset Management beneficially owns 11,162,809 shares of Miami International Holdings common stock, representing 12.2% of the outstanding shares as of April 27, 2026. This gives the investor a significant minority position with sole voting and dispositive power over those shares.

Who is the reporting person in this Miami International Holdings (MIAX) Schedule 13D/A?

The reporting person is Horizon Kinetics Asset Management LLC, a Delaware limited liability company and registered investment adviser managing various accounts that hold Miami International Holdings shares. Its parent, Horizon Kinetics Holding Corp, may be deemed to share beneficial ownership through its ownership of the adviser.

Why did Horizon Kinetics file Amendment No. 3 for Miami International Holdings (MIAX)?

Amendment No. 3 updates Horizon Kinetics’ beneficial ownership disclosure and related information. It restates identity and background details, confirms ownership of 11,162,809 shares (12.2%), describes investment purposes, and notes that Managing Director Eric Sites was elected to Miami International Holdings’ Board of Directors on April 24, 2026.

What are Horizon Kinetics’ intentions for its Miami International Holdings (MIAX) shares?

Horizon Kinetics states it acquired Miami International Holdings shares for investment purposes on behalf of managed accounts. It may periodically buy additional shares or sell existing holdings in open-market or privately negotiated transactions, influenced by client needs and regulatory or investment considerations.

Did Horizon Kinetics gain board representation at Miami International Holdings (MIAX)?

Yes. The filing notes that on April 24, 2026, Eric Sites, a Managing Director of Horizon Kinetics Asset Management, was elected to the Board of Directors of Miami International Holdings. This adds governance representation alongside the firm’s 12.2% beneficial ownership stake.

How does Horizon Kinetics describe its beneficial ownership of Miami International Holdings (MIAX)?

Horizon Kinetics Asset Management reports sole voting and dispositive power over 11,162,809 shares held in managed accounts. The firm expressly disclaims beneficial ownership beyond its and its clients’ pecuniary interest, clarifying the disclosure is for Sections 13(d) and 16 reporting purposes only.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)

MIAMI INTERNATIONAL HOLDINGS, INC.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

59356Q108

(CUSIP Number)

Jay Kesslen
470 Park Ave S, 8th Floor South
New York, NY, 07030
2128671176

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

04/24/2026

(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).

schemaVersion:

SCHEDULE 13D

CUSIP Number(s): 59356Q108

1

Name of reporting person

HORIZON KINETICS ASSET MANAGEMENT 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)

WC

5

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

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 11,162,809.00
8	Shared Voting Power
9	Sole Dispositive Power 11,162,809.00
10	Shared Dispositive Power

11

Aggregate amount beneficially owned by each reporting person

11,162,809.00

12

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

13

Percent of class represented by amount in Row (11)

12.2 %

14

Type of Reporting Person (See Instructions)

IA

Comment for Type of Reporting Person:

Horizon Kinetics Asset Management LLC ("HKAM") is a Delaware limited liability company and a wholly owned subsidiary of Horizon Kinetics Holding Corporation ("HKHC"), a Delaware corporation, and manages funds and accounts ("Managed Accounts") that hold securities of the Issuer. The following constitutes Amendment No. 3 to the schedule 13D filed by the undersigned. This Amendment No. 3 amends, supplements and to the extent inconsistent with, supersedes the initial Schedule 13D filed with the Securities and Exchange Commission on August 13, 2025, as amended by Amendment No. 1 filed November 24, 2025 and Amendment No. 2 filed April 14, 2026.

SCHEDULE 13D

CUSIP Number(s): 59356Q108

1

Name of reporting person

Horizon Kinetics Holding Corp

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)

AF

5

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

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 11,162,809.00
8	Shared Voting Power
9	Sole Dispositive Power 11,162,809.00
10	Shared Dispositive Power

11

Aggregate amount beneficially owned by each reporting person

11,162,809.00

12

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

13

Percent of class represented by amount in Row (11)

12.2 %

14

Type of Reporting Person (See Instructions)

HC

Comment for Type of Reporting Person:

HKHC is the parent company to HKAM, a registered investment adviser. HKHC may be deemed to beneficially own the 11,162,809 shares beneficially owned by HKAM. HKHC has not entered into any separate transactions in the shares following the filing of Amendment No. 3 other than those listed under Schedule A, which were effectuated by HKAM.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock
(b)	Name of Issuer: MIAMI INTERNATIONAL HOLDINGS, INC.
(c)	Address of Issuer's Principal Executive Offices: 7 ROSZEL ROAD, FIFTH FLOOR, SUITE 1-A, PRINCETON, NEW JERSEY , 08540.
Item 2.	Identity and Background
(a)	Item 2 (a) is hereby amended and restated in its entirety to read as follows: This Schedule 13D is being filed by HKAM, a Delaware limited liability company and a wholly owned subsidiary of HKHC, a Delaware corporation, with respect to the shares of common stock of the Issuer beneficially owned by Managed Accounts. On April 24, 2026, Eric Sites, who serves as a Managing Director of HKAM, was elected to the Board of Directors of the Issuer.
Item 4.	Purpose of Transaction
	Item 4 is hereby amended and restated in its entirety to read as follows: HKAM acquired the securities reported herein on behalf of the Managed Accounts for investment purposes. In the ordinary course of its business, HKAM intends to regularly review its equity interest in the Issuer and may, from time to time, acquire additional shares or other securities of the Issuer. While HKAM has no present intention to dispose of all or any portion of the shares beneficially owned by Managed Accounts, it may sell shares from time to time for a number of reasons, not limited to client requests, regulatory or investment limitations or other reasons. Any such sales of securities of the Issuer may be in the open market, privately negotiated transactions or otherwise. On April 24, 2026, Eric Sites, who serves as a Managing Director of HKAM, was elected to the Board of Directors of the Issuer.
Item 5.	Interest in Securities of the Issuer
(a)	Item 5 is hereby amended and restated in its entirety to read as follows: As of the close of business on April 27, 2026, HKAM beneficially owned 11,162,809 shares of common stock. The 11,162,809 shares beneficially owned represented 12.2% of the outstanding shares of common stock. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that HKAM is the beneficial owner of the common stock referred to herein for purposes of Sections 13(d) and or 16 of the Securities Exchange Act of 1934, or for any other purpose, and such beneficial ownership is expressly disclaimed, except to the extent of its pecuniary interest therein. On April 24, 2026, Eric Sites, who serves as a Managing Director of HKAM, was elected to the Board of Directors of the Issuer.
(b)	HKAM's Sole Power to vote is 11,162,809 shares.
(c)	Except as set forth in Schedule A attached hereto as Exhibit 99-a, HKAM has not effected any transaction in common stock in the past 60 days.
(d)	Except for Managed Accounts, who may have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, common stock, no other person is known by HKAM to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, any common stock that may be beneficially owned by HKAM.
(e)	N/A
Item 7.	Material to be Filed as Exhibits.
	Exhibit 99.1 - 60 Day Trading History

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.

HORIZON KINETICS ASSET MANAGEMENT LLC

Signature:	/s/ Jay Kesslen
Name/Title:	Jay Kesslen, General Counsel
Date:	04/28/2026

Horizon Kinetics Holding Corp

Signature:	/s/ Jay Kesslen
Name/Title:	Jay Kesslen, General Counsel
Date:	04/28/2026