The 1978 Major League Baseball season saw the New York Yankees defeat the Los Angeles Dodgers to win their second consecutive World Series, and 22nd overall, in a rematch of the prior season's Fall Classic. The Yankees overcame clubhouse turmoil, a mid-season managerial change, and a 14-game mid-July deficit in the American League East en route to the championship. All four teams that made the playoffs in 1977 returned for this postseason; none of the four would return to the postseason in 1979.
April 13 – Reggie Jackson hits a 3-run home run in the first inning of the New York Yankees' home opener. Jackson is showered with "Reggie Bar" candy bars, which had been given out free to fans in attendance. The Yankees defeat the Chicago White Sox, 4–2.
April 16 – St. Louis Cardinal Bob Forschno-hits the Philadelphia Phillies at Busch Stadium, striking out 3 and walking 2 in a 5–0 victory. It is the first of two no-hitters Forsch will throw in his career.
July 13 – Nolan Ryan of the California Angels and Steve Renko of the Boston Red Sox take no-hitters into the ninth inning of their respective games before both lose their no-hit bids. Ryan's Angels defeat the New York Yankees, 6–1, while Renko's Red Sox shut out the Oakland Athletics, 2–0.
July 17 – In the latest incident in their tumultuous relationship, Yankees manager Billy Martin suspends Reggie Jackson for five days after accusing Jackson of ignoring signs from the dugout. During the bottom of the 10th inning of a tie game against the Kansas City Royals, Martin gives Jackson the bunt sign. After Jackson fails miserably on his first attempt, Martin takes the sign off, but Jackson bunts again anyway. Jackson pops up to the catcher, Martin pinch hits for Jackson the next inning, and the Yankees go on to lose the game, 5–2. The loss drops the Yankees 14 games behind the first-place Boston Red Sox.
July 24 – In Kansas City, an anguished Billy Martin announces his resignation as Yankees manager. At the time, the defending champion Yankees are 52–42 and 10 games behind the first-place Boston Red Sox in the American League East. Martin's resignation comes one day after he said of right fielder Reggie Jackson and owner George Steinbrenner, "The two of them deserve each other. One's a born liar, and the other's convicted." Bob Lemon is named manager, but third base coach Dick Howser fills in for Martin that evening against the Royals.
July 29 – Before an Old Timers Day crowd of 46,711, Yankee Stadium announcer Bob Sheppard reveals that Billy Martin, who resigned as Yankees manager just five days earlier, will return as manager beginning in 1980, with Bob Lemon moving to the front office as GM. As it happens, the Yankees accelerate the timetable and Martin winds up taking over during the 1979 season.
August 1 – Pete Rose's 44-game hitting streak comes to an end in Atlanta. Rose goes 0–4 against Braves pitchers Larry McWilliams and Gene Garber, and strikes out in the ninth inning to end the game. Rose's streak is the second-longest in major league history, and he bats .385 (70 of 182) during the stretch.
August 20 – Los Angeles Dodgers teammates Steve Garvey and Don Sutton engage in a clubhouse brawl prior to the Dodgers' 5–4 win over the Mets at Shea Stadium. Garvey and Sutton had been feuding for some time, but public comments by Sutton about Garvey's clean-cut image sparked the brawl.
August 25 – Major league umpires stage a one-day strike in violation of their union contract. The league is forced to employ amateur umpires until a restraining order compels the striking umpires to return to work.
September 5 – The Montreal Expos defeat the Chicago Cubs, 10–9, in a nine-inning game that sees a major league record 45 players participate.
September 7 – 10 – The famed "Boston Massacre" occurs at Fenway Park. The first-place Boston Red Sox enter the four-game series against the second-place New York Yankees with a four-game lead in the AL East, down from 14 just seven weeks earlier. The Yankees pummel the Red Sox by scores of 15–3, 13–2, 7–0, and 7–4. The Yankees outscore the Red Sox 42 to 9 during the four-game sweep and find themselves atop the division for the first time all season.
September 15 – The Los Angeles Dodgers become the first team in major league history to draw 3 million fans in a season.
October 1 – Luis Tiant throws a two-hit shutout as the Boston Red Sox defeat the visiting Toronto Blue Jays, 5–0, to close the season on an eight-game winning streak. The win moves the Red Sox into a first-place tie and forces a one-game playoff against the New York Yankees at Fenway Park the following afternoon.
October 11 – The Dodgers take a 2–0 lead in the World Series with a 4–3 win in Game 2. The Dodgers' Ron Cey and the Yankees' Reggie Jackson drive in all the runs for their respective teams. Bob Welch saves the win for Burt Hooton in dramatic fashion, putting two runners on before striking out Jackson for the final out.