A self-adjusting spinner is provided that is capable of accommodating various pipe sizes without requiring the need for an operator to climb up the support mechanism and manually change the position of the drive assembly. The self-adjusting spinner includes a case having two pivotally connected members: a stationary case member and a moving case member. Upper and lower plates having gear racks are mounted on the stationary case member for moving a drive assembly horizontally across the case. The drive assembly includes a motor that drives gear sprocket through a drive shaft. The drive sprocket then drives a chain that rotates a drill pipe in an operative position relative to the case. The spinner also includes an adjusting assembly mounted on the case that moves the drive assembly along the gear rack upon the actuation of an adjustment sequence. When the adjustment sequence is initiated, the effective length of the chain is adjusted to accommodate drill pipes of varying diameters.