Chicago White Sox

Frank Thomas donned a White Sox uniform for 16 seasons, but now he's taking his former team to court ... TMZ Sports has learned the Hall of Famer filed a lawsuit against the club, insisting they're using his name to sell merch without permission!

Thomas filed suit last week in Illinois, and he's targeting not just the White Sox, but Nike and Fanatics, too ... and the beef is over the Sox "City Connect 2.0" jersey, which the team announced in April 2025.

In the complaint, Thomas claims his name and number 35 were featured on some of the uni tops that were put up for sale, despite never getting consent, according to Thomas.

The Big Hurt says the White Sox -- and the retailers -- likely raked in huge profits, while he says he received "no compensation" for the use of "Thomas" and his iconic number 35.