In baseball, a perfect game is a game in which one or more pitchers complete a minimum of nine innings with no batter from the opposing team reaching base safely. To achieve a perfect game, a team must not allow any opposing player to reach base by any means: no hits, walks, hit batsmen, catcher's interference, fielder's obstruction, and no fielding error, or uncaught third strike that allows a batter to reach base.

A perfect game, by definition, is also a no-hitter, and is also guaranteed to result in a win and a shutout if the game does not go into extra innings. In leagues that use a WBSC tiebreaker (including MLB since 2020), runners are placed on second base, and in some leagues, also on first base at the start of each half-inning during extra innings; this automatic runner would not cause a perfect game to be lost. Therefore, if the runner advances and scores without any batters reaching base (by means of stolen base, sacrifice, fielder's choice, etc.), and this turns out to be the winning run, then the losing team will still be credited with a perfect game, despite losing the game. A fielding error that does not allow a batter to reach base, such as a misplayed foul ball, does not spoil a perfect game. Games that last fewer than nine innings, regardless of cause, in which a team has no baserunners do not qualify as perfect games. Games in which a team reaches first base only in extra innings also do not qualify as perfect games.

The first known use of the term perfect game was in 1908; its current definition was formalized in 1991. In Major League Baseball (MLB), it has been achieved 24 times – 22 times since the modern era began in 1901, most recently by Domingo Germán of the New York Yankees on June 28, 2023, against the Oakland Athletics. Although it is possible for two or more pitchers to combine for a perfect game (which has happened 21 times in MLB no-hitters), every MLB perfect game so far has been thrown by a single pitcher. A combined perfect game occurred in Game 5 of the 2007 Japan Series of Nippon Professional Baseball.

As of 2024^[update], the Major League Baseball definition of a perfect game is largely an adjunct to the decision made by the major leagues' Committee for Statistical Accuracy on September 4, 1991, to redefine a no-hitter as a game in which the pitcher or pitchers on one team throw a complete game of nine innings or more without surrendering a hit. That decision removed a number of games that had long appeared in the record books: those lasting fewer than nine innings, and those in which a team went hitless in regulation but then got a hit in extra innings. The definition of perfect game was made to parallel this new definition of the no-hitter, in effect substituting "baserunner" for "hit". As a result of the 1991 redefinition, for instance, Harvey Haddix does not receive credit for a perfect game or a no-hitter for his performance on May 26, 1959, when he threw 12 perfect innings against the Milwaukee Braves before a batter reached in the 13th.

Since MLB implementation of the softball variant of the WBSC tie-breaker in 2020, the offensive team is awarded a free runner on second base each half-inning during extra innings. This itself would not end a perfect game, even if the runner scores or is erased on a double play. Another rule change effective for two seasons (2020–2021) stipulated that games that are part of doubleheaders last only seven innings. Such a game in which one team did not reach first base would not have been credited as a perfect game (similar to weather-shortened games). However, if such a doubleheader game were to have at least two extra innings and one team still did not reach first base, then the game would have been credited as a perfect game. During those two seasons, no potential perfect games were affected but there were two potential no-hitters affected.

Both rule changes were expected to be reversed prior to the 2022 season, but the international tiebreaker was permanently added to the Official rules of Major League Baseball regular season rules in February 2023. The WBSC tiebreaker is not used in postseason play.

The first known occurrence of the term perfect game in print was in 1908. I. E. Sanborn's report for the Chicago Tribune about Addie Joss's performance against the White Sox calls it "an absolutely perfect game, without run, without hit, and without letting an opponent reach first base by hook or crook, on hit, walk, or error, in nine innings". Several sources have claimed that the first recorded usage of perfect game was by Ernest J. Lanigan in his Baseball Cyclopedia, made in reference to Charlie Robertson's 1922 perfect game. The Chicago Tribune came close to the term in describing Lee Richmond's game for Worcester in 1880: "Richmond was most effectively supported, every position on the home nine being played to perfection." Similarly, in writing up John Montgomery Ward's 1880 perfect game, the New York Clipper described the "perfect play" of Providence's defense.

There has been one perfect game in the World Series, thrown by Don Larsen for the New York Yankees against the Brooklyn Dodgers on October 8, 1956. By coincidence, Larsen, and the catcher for that game, Yogi Berra, were in attendance when Yankee pitcher David Cone threw a perfect game in 1999. Larsen and Berra had been invited to throw and catch the ceremonial first pitch.