一人左投（Left-handed specialist），有时称作LOOGY（从英文的Lefty One Out Guy而来），是棒球术语，指专门对付左打者或较不擅长右打的左右开弓打者之中继左投，此类投手通常只投短暂局数，常只投一人次就换其他中继投手，并且极少面对纯右打者，因而得名。大部分的职业棒球队伍，特别是大联盟球队的牛棚中，都会有一位以上担任一人左投的投手。

一人左投出现的原因是因为右投较常见于场上，而预期以左投手较容易造成打者的不习惯而出局（即所谓的「左投克左打」）发展出的中继投手类型。

相对于一人左投，一人右投则远为少见，其原因有下面几点。虽然右投之于右打在球路上的混淆相当于左投对于左打，然而一般而言打击者会有70%~80%打席是面对右投，因此对于右打对于右投的习惯程度远高于左打之于左投。另外，大部分的左投为了增加对于左打面对左投经验不足所造成的压制性，会调整、练习使自己的球路更能混淆左打。虽然右投也可以将球路练习得更能混淆右打，但是考虑到右打面对大量右投所得到的经验，这样的作法效果通常并不显著。

In baseball, a left-handed specialist (also known as lefty specialist) is a relief pitcher who throws left-handed and specializes in pitching to left-handed batters, weak right-handed batters, and switch-hitters who bat poorly right-handed. Because baseball practices permanent substitution, these pitchers frequently pitch to a very small number of batters in any given game (often only one), and rarely pitch to strictly right-handed batters. Most Major League Baseball (MLB) teams have several left-handed pitchers on their rosters, at least one of whom is a left-handed specialist. A left-handed specialist is sometimes called a LOOGY (or Lefty One-Out GuY), a term that can be used pejoratively and which was coined by John Sickels.