Major League Baseball (MLB) annually honors its best relief pitchers in the American League (AL) and National League (NL) with the Mariano Rivera AL Reliever of the Year Award and Trevor Hoffman NL Reliever of the Year Award, respectively. The awards are named after former relievers Mariano Rivera and Trevor Hoffman, who played their entire careers in the respective leagues. First issued in 2014, the awards replaced the Delivery Man of the Year Award, which had been presented since 2005.

In baseball and softball, a relief pitcher or reliever is a pitcher who pitches in the game after the starting pitcher has been removed because of fatigue, ineffectiveness, injury, or ejection, or for other strategic reasons, such as inclement weather delays or pinch hitter substitutions. Relief pitchers are further divided informally into various roles, such as closers, setup men, middle relief pitchers, left/right-handed specialists, and long relievers. Whereas starting pitchers usually throw so many pitches in a single game that they must rest several days before pitching in another, relief pitchers are expected to be more flexible and typically pitch in more games with a shorter time period between pitching appearances but with fewer innings pitched per appearance. 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.