后援投手，指的是在棒球、垒球运动中，接替先发投手的继任投手，并大致分为布局投手、中继投手、终结者、一人左投、长中继等。

In baseball and softball, a relief pitcher or reliever is a pitcher who enters the game after the starting pitcher is removed due to injury, ineffectiveness, fatigue, ejection, or for other strategic reasons, such as being substituted by a pinch hitter. Relief pitchers are further divided informally into various roles, such as closers, set-up relief pitchers, middle relief pitchers, left/right-handed specialists, and long relievers. Whereas starting pitchers usually rest several days before pitching in a game again due to the amount of pitches thrown, relief pitchers are expected to be more flexible and typically pitch more games but with fewer innings pitched. A team's staff of relievers is normally referred to metonymically as a team's bullpen, which refers to the area where the relievers sit during games, and where they warm-up prior to entering the game.