Abstract: An indexing rotary actuator is provided for unidirectionally rotating an output shaft through a specific angle per each indexing operation accurately without cumulative error. A pair of symmetrical housing members are mounted together to form an actuator housing. Within the actuator housing are formed a main piston chamber and a clutch piston chamber. Within the clutch piston chamber, a pair of piston mounting rings are mounted opposing each other, wherein a pair of respective main pistons within the main piston chamber are affixed to outer diametrical surfaces of the piston mounting rings. Furthermore, a pair of clutch pistons are provided within the clutch piston chamber slidably supported along the output shaft within the clutch piston chamber. The main pistons are air or fluid powered to be rotatable about the annular main piston chamber.