The following are the baseball events of the year 1903 throughout the world.

• January 3 - Frank J. Farrell and Bill Devery pay $18,000 for the defunct American League franchise in Baltimore and relocate the team to New York and called them the Highlanders. The Highlanders would years later receive a new team name and become the New York Yankees.
• January 13 - Win Mercer, recently appointed player-manager of the Detroit Tigers, commits suicide on January 13, 1903 in San Francisco during a barnstorming tour. Mercer reportedly had a gambling problem.^[4]
• February 17 - The Brooklyn Superbas purchase the contracts of pitcher Rube Vickers and outfielder Harry Thielman from the Cincinnati Reds.
• March 7- The Detroit Tigers trade pitcher/infielder Kid Gleason to the New York Giants for second baseman Heinie Smith
• April 20- Future hall of fame pitcher Chief Bender makes his major league debut for the Philadelphia Athletics at the age of 18.
• May 6 – The Chicago White Stockings committed twelve errors, and the Detroit Tigers answered back with six of their own. The combined "18-E debacle" set a modern Major League record for the most errors (by two teams) in a single game.
• May 7 – Pittsburgh Pirates outfielder Fred Clarke hits for the cycle for the second time in his career. The Pirates lose to the Cincinnati Reds, however, 11–8.
• June 21 – Boston Americans outfielder Buck Freeman hits for the cycle in a 12–7 Boston win over the Cleveland Naps.
• June 25 – Wiley Piatt of the Boston Beaneaters became the only pitcher in the 20th century to lose two complete games in one day. Piatt allowed fourteen hits, while striking out twelve, en route to 1–0 and 5-3 St. Louis Cardinals victories.
• June 29 – Patsy Dougherty, outfielder for the Boston Americans, hits for the cycle against the Chicago White Stockings leading Boston to a 7–2 win.
• July 1- Cy Young drives in the lone run to lead Boston to a 1–0 victory over Chicago.
• July 2 - Jack Doscher, a pitcher for the Chicago Cubs, becomes the first son of a former major league player to play in the majors. His father, Herm Doscher, played from 1872-1882 as a journeyman outfielder/infielder.
• July 16 - The Detroit Tigers acquired Paddy Greene from the New York Highlanders in exchange for John Deering.
• August 17- The Detroit Tigers release second baseman Heinie Smith, whom they acquired via trade prior to the start of the season.
• September 3 – Cleveland Naps rookie Jesse Stovall tosses an 11-inning shutout, 1–0, over the Detroit Tigers. The feat still remains as the longest shutout ever for a major league pitching debut.
• September 18 – Chick Fraser pitches a no-hitter for the Philadelphia Phillies in the second game of a doubleheader against the Chicago Cubs. The Phillies win, 10–0.
• September 24 – Cleveland Naps third baseman Bill Bradley hits for the cycle against the Washington Senators in a 12–2 Cleveland win.
• October 1 – In Game 1 of the first modern World Series in Major League Baseball, Pittsburgh Pirates outfielder Jimmy Sebring becomes the first player to hit a home run in the World Series when he connects for a solo shot off of Boston's Cy Young in the seventh inning. The Pirates beat the Americans, 7–3.
• October 2 – Boston Americans outfielder Patsy Dougherty becomes the first player to hit multiple home runs in a World Series game when he hits solo home runs in the first and sixth inning of Game 2 in Boston's 3–0 win over the Pittsburgh Pirates.
• October 13 – The Boston Americans defeat the Pittsburgh Pirates, 3–0, in Game 8 of the first World Series. Boston wins the series, five games to three.

• January 12 – Win Mercer, 28, pitcher for four teams from 1894 to 1902, who posted two 20-win seasons and led the National League in games started, shutouts, and saves in the 1897 season.
• January 13 – Pete Conway, 36, pitcher who posted a 61–61 record for four teams from 1885 to 1889.
• February 6 – Hardie Henderson, 40, pitcher who went 81–121 with four teams between 1883 and 1888.
• February 11 – Sam McMackin, [?], pitcher who played with the Chicago White Sox and Detroit Tigers in the 1902 season.
• February 15 – Phil Reccius, 40, played third base for eight seasons, most notably for the Louisville Eclipse/Colonels.
• February 20 – Al Dwight, 47, pitcher for the 1884 Kansas City Cowboys.
• March 19 – John Conkey, 83, Boston businessman who co-founded the Boston Red Stockings of the National Association, predecessor of the modern Braves franchise, in 1871 and served as its president in 1871–1872.
• May 2 – Odie Porter, 25, pitcher who played briefly for the 1902 Philadelphia Athletics.
• May 3 – Count Sensenderfer, 55, played for the Philadelphia Athletics from 1871 to 1874. Later became a politician.
• May 13 – Thomas Lynch, 40, pitcher who played for the Chicago White Stockings in the 1884 season.
• May 16 – Chicken Wolf, 41, right fielder for 11 years, 10 with the Louisville Colonels.
• June 22 – Fatty Briody, 44, catcher for eight seasons from 1880 to 1888.
• July 1 – Jimmy Cooney, 37, shortstop for the Chicago Colts and Washington Senators National League teams from 1890 to 1892.
• July 2 – Ed Delahanty, 35, slugging left fielder since 1888, a three-time .400 hitter who ranked second only to Cap Anson in career hits; died after falling from a railroad trestle crossing the Niagara River.
• August 1 – Charlie Bohn, 47, outfielder/pitcher who played for the 1882 Louisville Eclipse.
• August 2 – Bill Sweeney, [?], pitcher/outfielder for the 1882 Philadelphia Athletics and the 1884 Baltimore Monumentals.
• August 21 – Andy Leonard, 57, left fielder for the original Cincinnati Red Stockings, the first fully professional baseball team.
• October 10 – John Valentine, 47, umpire from 1884 to 1888, who previously pitched for the 1883 Columbus Buckeyes.
• October 22 – Joe Yingling, 36, pitcher for the 1886 Washington Nationals.
• November 5 – Harrison Peppers, 37, pitcher for the Louisville Colonels during the 1894 season.
• November 12 – John Gilbert, 39, shortstop for the 1890 Pittsburgh Alleghenys.
• November 28 – Jack Easton, 38, pitcher who posted a 26–29 record in 76 games for the Columbus Solons, St. Louis Browns, and Pittsburgh Pirates from 1889 to 1894.
• December 30 – Dan Leahy, 33, shortstop for the 1896 Philadelphia Phillies.
• December 31 – Joe McGuckin, 41, outfielder for the 1890 Baltimore Orioles of the American Association.