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.

The 1903 World Series was the first modern World Series to be played in Major League Baseball. It matched the American League (AL) champion Boston Americans against the National League (NL) champion Pittsburgh Pirates in a best-of-nine series, with Boston prevailing five games to three, winning the last four. The first three games were played in Boston, the next four in Allegheny, and the eighth (last) game in Boston.