Second base, or 2B, is the second of four stations on a baseball diamond which must be touched in succession by a base runner in order to score a run for that player's team. A second baseman is the baseball player guarding second base. Also called second bagger, the second baseman often possesses quick hands and feet, needs the ability to get rid of the ball quickly, and must be able to make the pivot on a double play. In addition, second basemen are usually right-handed; only five left-handed throwing players have ever played second base since 1957. Second base is also known as the keystone sack. In the numbering system used to record defensive plays, the second baseman is assigned the number 4.

Good second basemen need to have very good range, since they have to field balls closer to the first baseman who is often holding runners on, or moving towards the base to cover. On a batted ball to right field, the second baseman goes out towards the ball for the relay. Due to these requirements, second base is sometimes a primarily defensive position in the modern game, but there are hitting stars as well.