The 1903 St. Louis Cardinals season was the team's 22nd season in St. Louis, Missouri and the 12th season in the National League. The Cardinals went 43–94 during the season and finished eighth in the National League.

The 1903 major league baseball season began on April 16, 1903. The regular season ended September 29, with the Pittsburgh Pirates and the Boston Americans as regular season champions of the National League and American League, respectively. The postseason began with Game 1 of the first modern World Series on October 1 and ended with Game 8 on October 13. The Americans defeated the Pirates, five games to three.