In baseball, a walk-off home run is a home run that ends the game. For a home run to end the game, it must be hit in the bottom of the final inning and generate enough runs to exceed the opponent's score. Because the opponent will not have an opportunity to score any more runs, there is no need to finish the inning and the teams can walk off the field immediately. The winning runs must still round all three bases and be counted at home plate. A variant of the walk-off home run, the walk-off grand slam, occurs when a grand slam exceeds the opponent's score in the bottom of the final inning and ends the game.

Although the concept of a game-ending home run is as old as baseball, the adjective "walk-off" attained widespread use only in the late 1990s and early 2000s.

The first known usage of the word in print appeared in the San Francisco Chronicle on April 21, 1988, Section D, Page 1. Chronicle writer Lowell Cohn wrote an article headlined "What the Eck?" about Oakland reliever Dennis Eckersley's unusual way of speaking: "For a translation, I go in search of Eckersley. I also want to know why he calls short home runs 'street pieces,' and home runs that come in the last at-bat of a game 'walkoff pieces' ..." Although the term originally was coined with a negative connotation, in reference to the pitcher (who must "walk off" the field with his head hung in shame),^[1] it has come to acquire a more celebratory connotation, for the batter who circles the bases with pride and with the adulation of the home crowd.

Jim Thome holds the MLB record for most career walk-off home runs with thirteen, the first being hit on June 15, 1994 and the last (which broke the previous record of twelve) on June 23, 2012. Most notably, he hit his 500th career home run for a walk-off home run.^[2][3]

The rules of baseball^[22] provide that:

• A batter is entitled to a home run only "when he shall have touched all bases legally." (Rule 5.05(a)(5); also 5.06(b)(4)(A))
• A batter is out, on appeal, for failing to touch each base in order or for passing a preceding runner. In some cases, all runs that score are negated. (Rule 5.09(b)(9), 5.09(c)(2) and 5.09(d))
• On a game-winning hit, a batter is credited for the full number of bases only if "the batter runs out his hit." (Rule 9.06(f))
• A game-winning home run is allowed to complete before the game ends, even if it puts the home team ahead by more than one run. (Rule 7.01(g)(3), Exception; also 9.06(g))

The first point above was problematic in the 1976 American League Championship Series between the New York Yankees and the Kansas City Royals. The Yankees and Royals entered the bottom of the ninth inning of the decisive fifth game with the score tied, 6–6; Mark Littell was the pitcher for Kansas City, and Chris Chambliss was the first batter for New York. Chambliss hit Littell's first pitch into the right field bleachers to win the game and the American League pennant for the Yankees. However, Yankees fans ran onto the field at Yankee Stadium to celebrate the victory, and prevented Chambliss from rounding the bases and touching home plate. Recognizing the impossibility of Chambliss successfully negotiating the sea of people who had been on the field, umpires later escorted Chambliss back out to home plate and watched as he touched it with his foot, thereby making the Yankees victory "official". (A comment to Rule 5.08(b) permits the umpires to award the run if fans prevent the runner from touching home plate.)

The third point above led to Robin Ventura's "Grand Slam Single" in Game 5 of the 1999 NLCS. In the bottom of the 15th inning, the New York Mets tied the score against the Atlanta Braves at 3–3. Ventura came to bat with the bases loaded, and hit a game-winning grand slam to deep right. Roger Cedeño scored from third and John Olerud appeared to score from second, but Todd Pratt,^[23] on first base when Ventura hit the home run, went to second, then turned around and hugged Ventura as the rest of the team rushed onto the field. The official ruling was that because Ventura never advanced past first base, it was not a home run but a single, and thus only Cedeño's run counted, making the official final score 4–3.

The fourth point above was not a rule prior to 1920; instead, the game ended at the moment the winning run scored. This rule affected the scoring of 40 hits, from 1884 to 1918, that would now be scored as game-winning home runs.^[24] Babe Ruth would have been credited with 715 career home runs had the modern rule been in effect in 1918; in a 10-inning game Ruth's fence-clearing, walk-off RBI hit was scored a triple because the game was deemed over when the lead baserunner reached home.^[25]

In the charts below, home runs that ended a postseason series are denoted by the series standing in bold. Home runs in which the winning team was trailing at the time are denoted by the final score in bold. Grand slams are denoted by the situation in bold.

Although the term itself would not be used until over 100 years later, Casey at the Bat, an 1888 poem by Ernest Thayer, features a potential walk-off home run. Although pessimistic at first, the home team's fans become more optimistic when their star, Casey, unexpectedly gets a chance to hit a walk-off three run home run. In the end they go home disappointed, however, when Casey strikes out rather than hitting the home run the fans expect.^[39]

• Game-winning RBI
• Homer in the Gloamin'
• Career record for walk-off home runs
• Chasing a target, the cricket equivalent