February – The 1990 Major League Baseball lockout begins. It lasts 32 days, and as a result virtually wipes out all of spring training and also pushes Opening Day back a week to April 9. In addition, the 1990 season has to be extended by three days in order to accommodate the normal 162-game schedule.
May 22 – Andre Dawson of the Chicago Cubs is intentionally walked by Cincinnati Reds' pitching five times. He is the first player to do so in Major League history.
June 6 – The highest-profile managerial firing of 1990 season happens when the New York Yankees fire Bucky Dent before a game against their rivals at Fenway Park, where he hit his famous three-run home run in a one-game playoff game in 1978, making Fenway Park the scene of his greatest moment as a player and worst moment as manager.
September 22 – Andre Dawson of the Chicago Cubs steals his 300th base in an 11–5 loss to the New York Mets, becoming only the second player in major league history with 300 home runs, 300 steals, and 2,000 hits. Willie Mays is the first, though they will later be joined by Barry Bonds.
September 25 – The Oakland Athletics secure their third straight American League West championship with a 5–0 shutout of the Royals in Kansas City. The A's would finish with the best record in baseball at 103–59, the third consecutive year they have done so.
October 3 – On the final day of the regular season, the Boston Red Sox clinch their third American League East title in five years with a 3–1 defeat of the Chicago White Sox. Tom Brunansky seals the win for the Red Sox with a sliding catch in the right field corner with the tying runs on base for the White Sox.
October 20 – The talk of an Oakland Athletics dynasty is proven premature, as the Cincinnati Reds beat Oakland 2–1 to complete one of the most stunning sweeps in World Series history. Series MVP José Rijo (2–0, 0.59 ERA) retires the last 20 batters he faces to give the Reds their first World Championship since 1976. Not joining the celebration at the end is Eric Davis, who ruptures his kidney diving for a ball during the game and is taken to the hospital. It will take Davis several years to fully recover.
December 6 – At Herman Darvick Autograph Auctions in New York City, Shoeless Joe Jackson's signature is sold for $23,100, the most money ever paid for a 19th- or 20th-century signature. Jackson, who could not read or write, copied the signature from one written out by his wife. The signature, which is resold within hours, was cut from a legal document.
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