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 and/or fatigue. Relievers are further divided informally into closers, middle relief pitchers, left-handed specialists, set-up pitchers and long relievers.

The importance of relief pitchers has increased significantly over the past decade. This change in mindsets can be seen by watching any major game, as well as by monitoring the salaries of such pitchers. In decades past, the relief pitcher was merely an ex-starter who came into a game upon the injury, ineffectiveness, or fatigue of the starting pitcher. The bullpen was for old stags, starters who had lost the ability to throw effectively. Many of these pitchers would be able to flourish in this diminished role. Those such as Dennis Eckersley, as with many others, actually prolonged their diminishing careers and often sparked them to new life. The added rest to their arms as well as the lessened exposure of their abilities became an advantage many would learn to capitialize on. Facing some batters only once a season, the opposition would have greater difficulty in preparation for the game. Still more, this would create another tangent to the game, in that more and more time would be set aside for the study of previous game films, a practice that over the years would greatly enhance the batting ability of most players. This focus on offense would create higher scoring, more exciting games, those that would draw larger crowds, as well as greater profits.

Secondly, the relief pitcher's position has become more of a career, rather than a degradation position. Many of today's top prospects are those of relief pitchers. Furthermore, the game today often calls for those in the bullpen to often step up and become occasional starters. Their limited exposure, as was stated before, often can give these chance starters an edge. This ability to fill starts effectively often leads to less down time of the main starters, as well as longer, healthier careers. For example, take the lengthy careers of such pitchers as Randy Johnson and Curt Schilling. These are just a few examples of those careers extended by the assistance of relief pitchers. Still further, many pitchers who started as relief pitchers have gone on to prosper and remain starters, like Pedro Martínez.

Another reason for the advancement of the skill in the relief positions is due to the game's ever expanding need for a psychological edge. Whether the game will admit it or not, the numbers game is always a factor in every outcome. A ball is a ball, a bat, a bat. Yet, the numbers game is constantly dictating the movement of the game, from inning to inning, from pitch to pitch. A batter who can't hit well off the movement of a left-hander's curveball will, undoubtedly face that left-hander in a late inning situation. In late innings such as the 7th, 8th, and 9th, many managers are known for changing pitchers at the rate of a dime a dozen. Such managers as Tony LaRussa are infamously known for changing pitchers for each batter.

Some of today's best known relief pitchers are Francisco Rodriguez, Eric Gagné, and Mariano Rivera. A young rookie reliever, Jonathan Papelbon, is currently the closer for the Boston Red Sox and appears to have a super-star career as a relief pitcher in the making. His future, however, is anything but definite. Four relievers are currently in the Baseball Hall of Fame – Hoyt Wilhelm, Rollie Fingers, Dennis Eckersley (though Eckersley had an impressive career as a starter berfore moving to the bullpen), and Bruce Sutter.

See also

• Rolaids Relief Man of the Year Award
• The Sporting News Reliever of the Year Award

Baseball pitching