The Kansas City Cowboys are admitted to the American Association.
January 27 – The Brooklyn Bridegrooms keep 5 players from the New York Metropolitans that they purchased last October and sell the rest, plus 4 of their own players, to the Kansas City Cowboys for $7,000.
February 2 – The Indianapolis Hoosiers announce that they will have 42 private boxes on top of their new grandstand. The boxes will only be available to season subscribers.
February 13 - The Kansas City Cowboys purchase the contract of Jumbo Davis from the Baltimore Orioles of the International League.
March 2 – The National League reverts to its original policy of a .50¢ admission price with no concessions given to individual clubs.
March 5 – The American Association votes to adopt the use of turnstiles at all parks in order to better control entry into games.
April 3 – The Chicago Cubs sell John Clarkson to the Boston Braves for $10,000.
April 29 Pitcher Charles Ferguson, a former 20 game winner for the Philadelphia Phillies, passes away at the age of 29. Just two season prior, Ferguson won 30 games for the Phillies.
May 15 – Philadelphia Athletics outfielder Harry Stovey hits for the cycle in a 12–3 win over the Baltimore Orioles.
May 21 - The Philadelphia Quakers purchase the contract of Ed Delahanty from Wheeling of the TriState Baseball League
May 27 – Adonis Terry pitches the second no-hitter of his career as the Brooklyn Bridegrooms defeat the Louisville Colonels, 4–0.
June 6 – Henry Porter of the Kansas City Cowboys tosses a no-hitter against the Baltimore Orioles in a 4–0 Kansas City win.
June 13 – Kansas City Cowboys second baseman Sam Barkley hits for the cycle against the Cincinnati Red Stockings. The Cowboys lose, however, 11–6.
June 21 – George Van Haltren pitches a 6-inning no-hitter to beat the Pittsburgh Pirates at West Side Park. The contest is cut short by rain. He strikes out 3 and walks 1.
July 4 – Albert Spalding, president of the Chicago White Stockings, has two ticket speculators arrested and jailed after they violate a city ordinance that prohibits the selling of tickets on the street.
July 7 – Dave Foutz of the Brooklyn Bridegrooms, former popular player with the St. Louis Browns, is carried off of the field by Browns fans after he gets the game-winning hit for Brooklyn in a game held in St. Louis.
July 13 – The Pittsburgh Alleghenys shut out the Boston Beaneaters in both games of a double-header, the first such occurrence in major league history.
July 14 – Adonis Terry tells his teammates that he heard Kansas City Cowboys manager Sam Barkley order substitute umpire Jim Donahue, who was the Cowboys regular catcher, to call a Bridegroom runner out in the 9th inning of a 5–4 game. The Grooms walked off the field in protest‚ forfeiting the game, resulting in a 9–0 score.
July 17 – Tommy McCarthy of the St. Louis Browns goes 5–5 at the plate to go along with 6 stolen bases in a game in which the Browns steal 15 bases in all.
July 26 – Ed Seward of the Philadelphia Athletics pitches a no-hitter in a 12–2 over the Cincinnati Red Stockings.
July 28 – Jimmy Ryan of the Chicago White Stockings hits for the cycle and pitches 7 innings in relief in a 21–17 win over the Detroit Wolverines. Ryan becomes the first player to hit for the cycle and pitch in the same game.
July 31 – Gus Weyhing of the Philadelphia Athletics pitches a no-hitter against the Kansas City Cowboys for the second no-hitter by an Athletics pitcher in 5 games.
July 31 – "Sliding" Billy Hamilton makes his major league debut with the Kansas City Cowboys.
August 7 – The American Association votes to allow teams to lower ticket prices to .25¢ and changes the gate guarantee for the visiting team from a percentage to a flat $130.
August 10 – Tim Keefe of the New York Giants wins his 19th consecutive game, setting a new major league record.
August 21 – The Detroit Wolverines blow a 2–0 lead by committing 6 errors in the final 2 innings to lose their 16th game in a row.
August 22 – Silver King of the St. Louis Browns loses a no-hitter in the 9th inning when 2 Browns outfielders let an easy fly ball drop between them.
August 22 – The Indianapolis Hoosiers attempt to play a "night" game at dusk by using natural-gas lights.
August 25 – New York Giants outfielder Mike Tiernan hits for the cycle as the Giants beat the Philadelphia Phillies, 7–0.
August 29 – Joe Quinn makes his first game as a Boston Beaneater a memorable one by homering in the bottom of the 9th to give Boston a 2–1 win over 30 game winner Tim Keefe.
September 4 – Pud Galvin became the first pitcher to win 300 games.
September 6 – The Indianapolis Hoosiers try again to play an evening game with the use of natural-gas lights. Unable to generate adequate lighting to play, the Hoosiers drop the idea for good.
September 7 – Dick Johnston of the Boston Beaneaters leads off the game with a home run for the 2nd straight game.
September 12 – The New York Giants are forced to forfeit their game with the Chicago White Stockings when they have no available substitutes to replace the injured Buck Ewing in the 5th inning.
September 15 – Ed Morris of the Pittsburgh Alleghenys pitches his 4th consecutive shutout, setting a National League record that will stand until 1968.
September 18 – Ben Sanders of the Philadelphia Quakers loses his perfect game with 1 out in the 9th inning when he allows a single to the Chicago White Stockings pitcher Gus Krock, a .164 career hitter.
September 20 – Cubs pitcher Frank Dwyer pitches a 3-hit shutout in his Major league debut, defeating the Washington Nationals 11–0 in the first game of a double-header at West Side Park.
September 27 – Ed "Cannonball" Crane pitches a 7-inning no-hitter for the New York Giants in their 3–0 win over the Washington Nationals.
October 1 – With darkness looming, the Indianapolis Hoosiers score 3 runs in the top of the 9th inning to take a 4–2 lead over the Washington Nationals when Washington catcher Connie Mack suddenly develops an "injury" to his finger. The delay causes the game to be called because of darkness with the score reverting to the last completed inning, resulting in a 2–1 Washington victory.
October 3 – The New York Giants and the St. Louis Browns each clinch their respective league pennants.
October 5 – Pud Galvin of the Pittsburgh Alleghenys becomes the first pitcher to record 300 career wins with a 5–1 victory over the Washington Nationals.
October 13 – The National League season comes to a close with the champion New York Giants setting a league attendance record by drawing 305,000 fans for the season.
October 16 – Rumours abound of the Detroit Wolverines dropping out of the National League and being replaced by the Cleveland Blues of the American Association after they sell star players Sam Thompson, Dan Brouthers and 30 game winner Pete Conway.
October 16 – The World Series opens with the New York Giants beating the St. Louis Browns 2–1.
October 17 – The Browns even the series by taking a 3–0 victory over the Giants.
October 18 – New York wins Game 3 4–2 over the Browns.
October 19 – New York takes a 3–1 series advantage with a 6–3 win over St. Louis.
October 20 – The Giants score 5 runs in the bottom of the 8th inning to take a 6–4 victory and a 4–1 lead in the series.
October 22 – New York wins again in Game 6 by a score of 12–5.
October 24 – The Browns get a must-win by scoring 4 in the 8th to beat the Giants 7–5.
October 25 – The New York Giants clinch the series with an 11–3 win over the St. Louis Browns. The final 2 games will be played for revenue purposes with St. Louis winning both contests for an overall series result of 6 games to 4 in favor of the Giants.
November 10 – A new club is organized in Detroit to compete in the International Association during the next season, in order to take the place of the disbanded Detroit Wolverines, who finished in fifth place in the National League this past season. The Wolverines sell off their stars, including Sam Thompson to the Philadelphia Quakers, as well as the so-called Big Four Dan Brouthers to the Boston Beaneaters, Hardy Richardson to the Boston Reds, and Jack Rowe and Deacon White to the Pittsburgh Alleghenys.
November 20 – The Joint Rules Committee reduces the number of balls needed for a walk to 4. With the 4 ball, 3 strike at-bat and overhand pitching rules now in place, baseball in 1889 will be played very similar to the game of today.
November 21 – The National League formally admits the Cleveland Blues from the American Association. The Blues are the 2nd team to leave the AA for the NL, following the Pittsburgh Alleghenys who made the switch after the 1886 season.
November 22 – The National League adopts a five-tier salary structure based on a player's on-field abilities and off-field personal habits, with the salary scale ranging from $1,500–2,500 in each tier. The Brotherhood of the Professional Baseball Players is incensed by the classification system and it will be the impetus for the organization of the Players' League in 1890.
November 23 – The World Champion New York Giants announce the sale of star player and leader of The Brotherhood of Professional Baseball Players, John Montgomery Ward, to the Washington Nationals for $12,000. The deal will fall through after Ward refuses to abide by the sale.
December 5 – The Columbus Solons are admitted to the American Association to replace the departed Cleveland Blues.