弹指球（英语：Knuckleball，knuckler），又称指关节球、旋转球、魔速球，棒球中的一种球路，是一种特殊变化球，球速不快，其重点在于尽量减少球在飞行运程中的旋转，造成球出现不稳定、无法预测的运动。投球时，挥臂不必非常用力，主要是靠着手指在伸直时的弹力把球弹出去，因此也称为弹指球。由于弹出的球几乎不会有旋转，所以会产生相当多的紊流，因此球会飞往何处无法预知，就如同飞舞的蝴蝶一般，故名为蝴蝶球。

蝴蝶球的握法有很多种，有些投手将手指弯起来用指甲抵着球，还有些则是用指尖扣着球，再用其他剩下的手指把球握住。

A knuckleball or knuckler is a baseball pitch thrown so as to minimize the spin of the ball in flight, causing an erratic, unpredictable motion. The air flow over a seam of the ball causes the ball to transition from laminar to turbulent flow. This transition adds a deflecting force on the side of the baseball. This makes the pitch difficult for batters to hit, but also difficult for pitchers to control and catchers to catch; umpires are challenged as well, as the ball's irregular motion through the air makes it harder to call balls and strikes.