A first baseman, abbreviated 1B, is the player on a baseball or softball team who fields the area nearest first base, the first of four bases a baserunner must touch in succession to score a run. The first baseman is responsible for the majority of plays made at that base. In the numbering system used to record defensive plays, the first baseman is assigned the number 3.

Baseball is a bat-and-ball sport played between two teams of nine players each, taking turns batting and fielding. The game occurs over the course of several plays, with each play generally beginning when a player on the fielding team, called the pitcher, throws a ball that a player on the batting team, called the batter, tries to hit with a bat. The objective of the offensive team is to hit the ball into the field of play, away from the other team's players, allowing its players to run the bases, having them advance counter-clockwise around four bases to score what are called "runs". The objective of the defensive team is to prevent batters from becoming runners, and to prevent runners' advance around the bases. A run is scored when a runner legally advances around the bases in order and touches home plate.