The American Association and the National League, along with the Northwestern League, sign the Tripartite Agreement (also known as the National Agreement). This agreement binds the leagues to respect each other's valid player contracts as well as increasing the size of the reserve list from 6 to 11 players. This leads to relative harmony among the leagues until the Players' League wars of 1889–1890.
March 14 – The Peoria Club of the Northwestern League makes a motion to ban blacks, a move directly aimed at Toledo's star catcher, Moses Fleetwood Walker. After heated discussion, the motion is withdrawn and Walker remains eligible to play.
March 30 – Charles Fowle, one of the original founders of the National League, and secretary of the St. Louis Brown Stockings from 1875–1877, dies in St. Louis.
March 31 – The nation's oldest baseball club, the Olympic Town-Ball Club of Philadelphia, marks its 50th anniversary.
April 15 – Francis Richter publishes the first issue of Sporting Life which will grow into the leading weekly publication for baseball information and run continuously until 1917.
April 24 – Terry Larkin, a pitcher who has not played in the majors since 1880, shoots his wife and a policeman, then tries to kill himself. He attempts suicide the next day and fails again. Both his wife and the police officer survive as well and Larkin will play in 40 games for the Richmond Virginians in 1884.
May 1 – In their inaugural National League game, the New York Gothams defeat the Boston Beaneaters 7-5 in front of 15,000 fans, who include President Ulysses S. Grant. The Philadelphia Quakers, also making their NL debut, lose 4-3 to the Providence Grays.
May 3 – John Montgomery Ward becomes the first pitcher to hit 2 home runs a game as his New York Gothams defeat the Boston Beaneaters 10-9.
May 13 – In what was still a very rare occurrence, neither team commits an error as the St. Louis Browns defeat the Louisville Eclipse 4-3.
May 28 – Fort Wayne and Indianapolis play the first of 2 games under electric lights.
May 30 – Several of the American Association teams play a Memorial Day double-header in 2 different cities. At one point, there is an American Association game being played at the Polo Grounds on the New York Metropolitans field and a National League game being played at the Polo Grounds on the New York Gothams field where the outfield fences back up to one another.
June 9 – The Philadelphia Quakers receive special permission from the National League to lower their ticket prices to 25¢ per game in order to compete with the Philadelphia Athletics of the American Association. The Quakers average game attendance quadruples for the remainder of the season.
June 16 – The New York Gothams introduce ladies day, where all females are admitted free without restriction. This idea will remain a staple of major league baseball for nearly 100 years.
June 28 – Providence Grays player Joe Mulvey is shot in the shoulder while leaving the playing field at Messer Street Grounds in Providence. The shooter, James Murphy, was actually aiming for Mulvey's teammate, Cliff Carroll after Carroll had drenched Murphy with a hose. Within a month, Mulvey would be sold to the Philadelphia Quakers.
July 3 – The Chicago White Stockings set a major league record with 14 doubles, including 4 each by Cap Anson and Abner Dalrymple, in a 31–7 pasting over the Buffalo Bisons. Chicago also set a Major League record for the most at bats by one team in a regulation nine-inning game, with 66.
July 4 – Tim Keefe of the New York Metropolitans wins both games of a double-header over the Columbus Buckeyes while giving up a combined 3 hits in the 2 games.
July 25 – Charles Radbourn throws a no-hitter for the Providence Grays.
July 26 – Joe Gephardt of the Louisville Eclipse is forced out of action due to temporary paralysis. Gephardt will recover and play again within 2 weeks.
July 28 – The first recorded game is played in Hawaii.
July 30 – Player/manager Lon Knight of the American Association Philadelphia Athletics hits for the cycle. Philadelphia defeats the Pittsburgh Alleghenys, 17-4.
August 7 – The Providence Grays fall out of 1st place for good in their 6–4 loss to the Boston Beaneaters. For the 2nd straight season, Providence will lead the league for the majority of the season and not win the pennant.
August 11 – The Boston Beaneaters turn a triple play when catcher Mike Hines catches a muffed pop-up and catches the base-runners off guard.
August 11 – Fred Thayer, patent-holder for the invention of the catching mask, and George Wright sue Wright's former teammate Albert Spalding's sporting goods company for patent infringement. Spalding will ultimately be forced to pay Thayer royalties in the case.
August 20 – The Pittsburgh Alleghenys fine George Creamer, Mike Mansell and Billy Taylor $100 each and indefinitely suspend all 3 players for drunkenness.
August 21 – The Providence Grays defeat the Philadelphia Quakers 28-0 in the most lopsided shutout game in major league history.
August 29 – John Stricker of the Philadelphia Athletics gets 4 hits in a game against the Louisville Eclipse but sets a record by getting picked off 3 times by Eclipse pitcher Guy Hecker. Benny Kauff of the New York Giants will tie Stricker's record in 1916.
September 6 – The Chicago White Stockings set a major league record by scoring 18 runs in the 7th inning in their 26–6 victory over the Detroit Wolverines. Tom Burns goes 3-3 with 2 doubles and a home run in the outburst, setting records for most extra-base hits and runs scored in one inning by a player.
September 10 – Long John Reilly of the Cincinnati Red Stockings hits 2 inside the park home runs in a 12–6 win.
September 12 – The Union Association is officially formed in Pittsburgh
September 12 – Long John Reilly hits for the cycle and collects 6 hits in all, while teammate Hick Carpenter also garners 6 hits, as the Cincinnati Red Stockings pound the Pittsburgh Alleghenys 27-5.
September 13 – Hugh Daily of the Cleveland Blues pitches a no-hitter.
September 19 – Long John Reilly hits for the cycle for the 2nd time in a week as the Cincinnati Red Stockings beat the Philadelphia Athletics 12-3.
September 27 – The Boston Red Stockings clinch the National League pennant with a 4–1 win over the Cleveland Blues.
September 28 – The Philadelphia Athletics clinch the American Association flag with a 7–6 victory over the Louisville Eclipse.
November 22 – New York Gothams owner John B. Day proposes a resolution to prohibit a team from signing a player who has broken the reserve clause of his contract. This resolution, eventually adopted by both the American Association and National League, effectively changes the reserve clause from a device to protect owners from their own greediness to a vindictive weapon to be used against uncooperative players.
November 24 – The American Association agree to expand to 12 teams by admitting the Brooklyn Atlantics, Indianapolis Hoosiers, Toledo Blue Stockings and Washington Nationals.