The Home Run Derby, officially known as the T-Mobile Home Run Derby for sponsorship reasons, is an annual home run hitting competition in Major League Baseball (MLB) customarily held the day before the MLB All-Star Game, which places the contest on a Monday in July. In the context of the competition a "home run" consists of hitting a baseball in fair territory out of the playing field on the fly. It differs from a home run hit during legal gameplay in that the batter is not competing against a pitcher and a defensive team attempting to make an out. In the Home Run Derby, all pitches are purposefully thrown slowly and at a closer range than the official 60 feet 6 inches (18.44 m) distance, usually by a coach behind a pitching screen. In addition, like batting practice, the batter remains in the batter's box after each swing and does not run, nor circle the bases to score a run. In 2023, Julio Rodríguez set the single-round home run record with 41 homers, topping Vladimir Guerrero Jr.'s 40 home runs in 2019.

The batter also does not have to conform to the usual league uniform standards, and as all pitches are tossed in the strike zone without any threat of beanballs, may choose to wear a regular baseball cap instead of a batting helmet. As the event traditionally takes place at sunset where the sun is of no factor to the batter, they can choose to wear their hat casually and backwards.

Since the inaugural derby in 1985, the event has seen several rule changes, evolving from a short-outs-based competition to multiple rounds, and eventually a bracket-style timed event.

T-Mobile has been the title sponsor of the Home Run Derby since 2021.

The inspiration for the event was a 1960 TV series called Home Run Derby. The televised event included baseball legends Hank Aaron, Mickey Mantle, and Willie Mays. The show ran for 26 episodes with the winner receiving $2,000.

The event has grown significantly from its roots in the 1980s, when it was not televised. Prior to 1991, the Home Run Derby was structured as a two-inning event with each player receiving five outs per inning, allowing for the possibility of ties. It is now one of the most-watched events broadcast on ESPN.

In 2000, a "match play"-style format was instituted for the second round. The player with the most home runs in the first round faced the player with the least among the four qualifying players, as did the players with the second- and third-most totals. The contestant who won each matchup advanced to the finals. This format was discontinued after the 2003 competition.

The field of players selected currently consists of four American League players and four National League players. The first Derby in 1985 featured five from each league, and the 1986 and 1987 events featured three and two players from each league, respectively. In 1996, the field was again expanded to ten players, with five from each league (though in 1997, the AL had six contestants to the NL's four).