The Aurora, Ont., native is entering his ninth season with the club, having played 530 games with 132 goals and 328 assists.
Thomas was also part of the 2019 Blues squad that won the Stanley Cup, when he appeared in 21 post-season games as a rookie.
Last season amid trade buzz, Thomas registered 25 goals and 39 assists in 64 games. He set career highs in goals (26) and points (86) during the 2023-24 season.
Thomas previously served as an assistant captain. Brayden Schenn, who had been captain since 2023-24, was traded to the New York Islanders midway through last season.