先发投手（英文：Starter、Starting Pitcher，缩写：SP）指在棒球比赛中第一位上场的投手，通常负担较多局数以及球队的胜败责任。投手投满5局且下场前球队领先即可获得胜投候选资格。通常先发投手都以体能较佳，续航力较久的投手为主，约5天上场一次。由于先发投手着重于长局数之平均压制能力，而且先发投手多半要负担至少五局的投球局数，为求有效率地完成至少五局的责任局数，先发投手除体能及续航力外，控球及配速的能力往往会是被要求的重点。美国职棒大联盟每队通常会有先发轮值名单，共五个投手。牛棚中，负责在先发投手投换后负责投球的称为中继投手或者是后援投手。

In baseball or softball, a starting pitcher or starter is the first pitcher in the game for each team. A pitcher is credited with a game started if they throw the first pitch to the opponent's first batter of a game. A pitcher who enters the game after the first pitch of the game is a relief pitcher. Starting pitchers are expected to pitch for a significant portion of the game, although their ability to do this depends on many factors, including effectiveness, stamina, health, and strategy.