Kenley Geronimo Jansen is a Curaçaoan professional baseball pitcher for the Boston Red Sox of Major League Baseball (MLB). He has previously played in MLB for the Los Angeles Dodgers and Atlanta Braves.

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.