A relief pitcher or reliever is a baseball or softball pitcher who enters the game after the starting pitcher is removed due to injury, ineffectiveness or fatigue. Relievers are further divided informally into closers, middle relief pitchers, left-handed specialists and set-up pitchers.

A relief pitcher generally has two good pitches – a fastball and some other pitch such as a slider, screwball, or changeup. Because of their limited pitch selection, batters tend to adjust quickly which limits relievers' effectiveness in repeat at-bats. Therefore, they often only pitch one or two innings.

In the early years of the sport, relief pitchers were generally former starting pitchers who no longer had the endurance to throw for several innings. But gradually over the period between the 1930s and 1960s, relief pitchers became a specialized position.

Some of today's best known relief pitchers are John Smoltz, Éric Gagné, and Mariano Rivera. Three relievers are currently in the Baseball Hall of Fame – Hoyt Wilhelm, Rollie Fingers, and Dennis Eckersley.