二垒手（Second Baseman，通常简写为2B）是在棒球或垒球比赛中，负责防守二垒的球员。其职责在于接捕一、二垒间的击球，以及接捕守备球员的传球来促使击球员或跑垒员出局。

二垒手通常守备位置在一、二垒之间。是个需要灵活身手的守备位置。平时除了接捕击球，也时常必需跑动到二垒以接捕其他守备球员的传球，与游击手相似，由于现代棒球左手击球员的增加，二垒手的重要性与游击手不相上下了。二垒手及游击手皆需担任外野转传球的中继角色,在与游击手搭配的双杀守备中,6-4-3及4-6-3的状况,二垒手接球转传的难度皆比游击手高。

In baseball and softball, second baseman is a fielding position in the infield, between third and first base. The second baseman often possesses quick hands and feet, needs the ability to get rid of the ball quickly, and must be able to make the pivot on a double play. In addition, second basemen are usually right-handed; only four left-handed throwing players have ever played second base since 1950. In the numbering system used to record defensive plays, the second baseman is assigned the number 4.