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 (home of the Pirates), and the eighth (last) game in Boston.

Pittsburgh pitcher Sam Leever injured his shoulder while trap shooting, so his teammate Deacon Phillippe pitched five complete games. Phillippe won three of his games, but it was not enough to overcome the club from the new American League. Boston pitchers Bill Dinneen and Cy Young led Boston to victory. In Game 1, Phillippe struck out ten Boston batters. The next day, Dinneen bettered that mark, striking out 11 Pittsburgh batters in Game 2.

Honus Wagner, bothered by injuries, batted only 6-for-27 (.222) in the Series and committed six errors. The shortstop was deeply distraught by his performance. The following spring, Wagner (who in 1903 led the league in batting average) refused to send his portrait to a "Hall of Fame" for batting champions. "I was too bum last year", he wrote. "I was a joke in that Boston-Pittsburgh Series. What does it profit a man to hammer along and make a few hits when they are not needed only to fall down when it comes to a pinch? I would be ashamed to have my picture up now."^: 138

Due to overflow crowds at the Exposition Park games in Allegheny City, if a batted ball rolled under a rope in the outfield that held spectators back, a "ground-rule triple" would be scored. 17 ground-rule triples were hit in the four games played at the stadium.

In the series, Boston came back from a three games to one deficit, winning the final four games to capture the title. Such a large comeback would not happen again until the Pirates came back to defeat the Washington Senators in the 1925 World Series, and has happened only 11 times in baseball history. (The Pirates themselves repeated this feat in 1979 against the Baltimore Orioles.) Much was made of the influence of Boston's "Royal Rooters", who traveled to Exposition Park and sang their theme song "Tessie" to distract the opposing players (especially Wagner). Boston wound up winning three out of four games in Allegheny City.

Pirates owner Barney Dreyfuss added his share of the gate receipts to the players' share, so the losing team's players actually finished with a larger individual share than the winning team's.

The Series brought the new American League prestige and proved its best could beat the best of the National League, thus strengthening the demand for future World Series competitions.