The following are the baseball events of the year 1891 throughout the world.
World Series: Boston (NL) declined to meet Boston (AA)
Major league baseball final standings
National League final standings
American Association final standings
National League statistical leaders
American Association statistical leaders
- February 6 – The New York Giants' salary list is leaked to the press. It shows a total player payroll of $54,600 with Buck Ewing's $5,500 salary topping the scale.
- April 11 – Clark Griffith‚ 21 years old‚ makes his Major League debut‚ pitching for the St. Louis Browns to a 13–5 victory over the Cincinnati Kelly's Killers. After spending much of 1892 and 1893 in the minor leagues‚ Griffith will return to remain active in the majors as a pitcher‚ manager‚ and club owner until his death in 1955.
- April 22 – In the first game at the third Polo Grounds, Boston beats the New York Giants, 4-3.
- May 1 – Cleveland's League Park opens with 10,000 fans on hand to see pitcher Cy Young beat Cincinnati, 12-3.
- May 14 – Charles Radbourn of the Cincinnati Reds records his 300th career win.
- May 22 – At Cincinnati's League Park, Mickey Welch of the New York Giants hits into a game-ending triple play. Batting with the bases loaded in the ninth inning, Welch lines one to shortstop Germany Smith, who catches the ball and tosses it to second baseman, Bid McPhee, who tags the runner caught off second, Charley Bassett. McPhee then relay the ball to first baseman, John Reilly, who toes the sack to retire the runner, Artie Clarke, who was running between bases. The Reds won 8–3.
- June 22 – Tom Lovett throws a no-hitter as the Brooklyn Grooms defeat the New York Giants, 4-0.
- October 4 – On the final day of the American Association season, Ted Breitenstein of the St. Louis Browns throws a no-hitter against the Louisville Colonels, in an 8–0 Browns win. It is Breitenstein's first major league start. He faced the minimum number of batters, 27, allowing just one base on balls. It was also the last no-hitter thrown in the American Association, as the league folded following the season.
- November 26 – A series for the championship of the Pacific Coast begins between the champions of the California League (the San Jose Dukes) and the Pacific Northwest League pennant winners (the Portland Webfeet). San Jose wins the opener, 8–6. The series will last until January 10 with San Jose winning 10 games to 9. All the games are played in San José, California.
- January 13 – Joe Connors, age unknown, pitched 3 games in 1884 in the Union Association.
- February 6 – Tom Healey, 37?, pitcher in 1878.
- February 25 – Jeremiah Reardon, 22?, pitcher who appeared in 2 games in 1886.
- April 14 – Frank Bell, 27?, played for the 1885 Brooklyn Grays.
- May 20 – Jim Fogarty, 27, utility player from 1884–1890. Led the National League in stolen bases with 99 in 1889.
- May 21 – Jim Whitney, 33, pitcher who had five 20-win seasons, including 37 for 1883 Boston champions; led NL in wins, games and innings as 1881 rookie, in strikeouts in 1883; good hitter also played center field, batted .323 in 1882.
- June 10 – Jerry Dorgan, 34?, reserve player from 1880–1885.
- July 2 – John Cassidy, 34?, right fielder for five teams who batted .378 for the 1877 Hartford Dark Blues.
- July 14 – Bill Crowley, 34, outfielder from 1875–1885.
- July 29 – Steve Matthias, 31?, shortstop for the 1884 Chicago Browns/Pittsburgh Stogies of the Union Association.
- August 25 – Jerry Sweeney, 31?, 1st baseman for the 1884 Kansas City Cowboys.
- August 28 – Joe Miller, 41, 2nd baseman who played from 1872–1875.
- October 11 – Will Smalley, 20, 3rd baseman for the 1890 Cleveland Spiders.
- October 14 – Larry Corcoran, 32, pitcher who won 175 games for the Chicago White Stockings from 1880 to 1885, led NL in wins, strikeouts and ERA once each; first pitcher to coordinate signals with his catcher, threw three no-hitters.
- October 21 – Ed Daily, 29, pitcher from 1885–1891. Won 26 games in 1885.
- November 19 – Ernie Hickman, 35?, starting pitcher for the Kansas City Cowboys of the Union Association in 1884.