On July 31, 2026, Interactive Brokers Group, Inc. (the "Company") filed a Prospectus Supplement under Rule 424(b)(5) to register up to 920,000 shares of the Company's common stock from its shelf Registration Statement on Form S-3 (Registration Statement No. 333-297857) (the "Registration Statement") filed with the SEC on July 31, 2026. The legal opinion letter of Dechert LLP, counsel to the Company, regarding the validity of the shares is filed as Exhibit 5.3 to this Current Report on Form 8-K. The legal opinion letter is also hereby incorporated by reference into the Registration Statement, as Exhibit 5.3 thereto.