An improved rotary drive sprinkler is provided for driving a sprinkler spray head in a rotary stepping motion to deliver irrigation water over a prescribed terrain area. The rotary sprinkler comprises a pop-up stem assembly carrying the spray head and movable within a sprinkler housing between a normal retracted position and an elevated spraying position when water under pressure is supplied to the sprinkler housing. During operation, the water flowing through the sprinkler housing powers a piston drive assembly including a spring-loaded drive piston which is alternately subjected to a predetermined pressure differential and normalized pressure to displace the piston in a reciprocating manner. The drive piston is coupled to a motion converter assembly for converting the reciprocal motion of the drive piston to an oscillatory rotary motion which is coupled in turn via a reversible one-way clutch assembly to the pop-up stem assembly. The clutch assembly can be set to rotate the pop-up stem assembly and spray nozzle carried thereby in one direction in a series of small rotational steps through continuous full-circle rotation or, in the alternative, reversible rotation between adjustably set end limits of a selected arcuate path.