A third baseman, abbreviated 3B, is the player in the sport of baseball whose responsibility is to defend the area nearest to third base, the third of four bases a baserunner must touch in a counterclockwise succession in order to score a run. In the numbering system used to record defensive plays, the third baseman is assigned the number 5.

The third baseman requires good reflexes in reacting to batted balls, as they are often the closest infielder (roughly 90-120 feet) to the batter. They also need a strong arm, since they have the farthest to throw to first base after fielding ground balls, and sometimes must throw quickly to second base in time to start a double play. The third baseman must also field fly balls in fair and foul territory.

Third base is known as the "Hot Corner", since most right-handed hitters will tend to hit the ball hard in this direction. In this situation, the third baseman needs fine eyes-glove coordination and quick reactions in order to catch hard line drives. And third basemen often must begin in a position even closer to the batter if a bunt is expected, creating a hazard if the ball is instead hit sharply. As with middle infielders, right-handed players are standard at the position because they do not need to turn their body before throwing across the infield to first base. Some third basemen have been converted from middle infielders or outfielders since the position does not require them to run as fast.

Expectations of how well a third baseman should be able to hit have varied a great deal over time; in the early years of the sport, these expectations were similar to those for shortstops, the third baseman being merely the less skilled defensive player. Players who could hit with more ability often were not suited for third base, either because they were left-handed or because they were not mobile enough for the position. However, the beginning of the live-ball era in the 1920s created a greater demand for more offense, and third basemen have since been expected to hit either for a high average (.290 or better) or with moderate to substantial power.

With the notable exception of John McGraw, surprisingly few third basemen have gone on to have successful managing careers, with Jimmy Dykes and Negro Leaguer Dave Malarcher being perhaps the next most prominent managers who began their careers at third base.

Significant third basemen

Baseball Hall of Fame members

• Frank Baker
• Wade Boggs
• George Brett
• Jimmy Collins
• Ray Dandridge (Negro Leagues)
• Judy Johnson (Negro Leagues)
• George Kell
• Freddie Lindstrom
• Eddie Mathews
• John McGraw (primarily elected as a manager)
• Paul Molitor (longtime designated hitter who played primarily 3B on the field)
• Brooks Robinson
• Mike Schmidt
• Pie Traynor

Multiple Gold Glove winners

• Brooks Robinson - 16
• Mike Schmidt - 10
• Buddy Bell - 6
• Scott Rolen - 6
• Robin Ventura - 6
• Ken Boyer - 5
• Doug Rader - 5
• Ron Santo - 5
• Eric Chavez - 4
• Gary Gaetti - 4
• Matt Williams - 4

Other notable stars

• Sal Bando
• Ken Caminiti
• Ron Cey
• Harlond Clift
• Lave Cross
• Darrell Evans
• Heinie Groh
• Stan Hack
• Willie Kamm
• Ken Keltner
• Arlie Latham
• Bill Madlock
• Dave Malarcher (Negro Leagues)
• Ollie Marcelle (Negro Leagues)
• Graig Nettles
• Terry Pendleton
• Tim Wallach
• Jud Wilson (Negro Leagues)
• Eddie Yost