The 1903 Boston Americans season was the third season for the professional baseball franchise that later became known as the Boston Red Sox. The Americans finished first in the American League (AL) with a record of 91 wins and 47 losses, 14+1⁄2 games ahead of the Philadelphia Athletics. Boston went on to participate in the first World Series held between the AL and National League (NL) champions. The Americans won the 1903 World Series in eight games over the Pittsburgh Pirates. The team was managed by Jimmy Collins and played its home games at Huntington Avenue Grounds.

The Boston Red Sox are an American professional baseball team based in Boston. The Red Sox compete in Major League Baseball (MLB) as a member club of the American League (AL) East Division. Founded in 1901 as one of the American League's eight charter franchises, the team's home ballpark has been Fenway Park since 1912. The "Red Sox" name was chosen by the team owner, John I. Taylor, c. 1908, following the lead of previous teams that had been known as the "Boston Red Stockings", including the Boston Braves. The team has won nine World Series championships, tied for the third-most of any MLB team, and has played in 13 World Series. Their most recent World Series appearance and win was in 2018. In addition, they won the 1904 American League pennant, but were not able to defend their 1903 World Series championship when the New York Giants refused to participate in the 1904 World Series.