A third baseman, abbreviated 3B, is the player in baseball whose responsibility is to defend the area nearest to third base — the third of four bases a baserunner must touch in succession to score a run. In the scoring 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 he is often the closest infielder (roughly 90–120 feet) to the batter. The third base position requires a strong arm, as the third baseman often makes long throws to first base. The third baseman 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.