A rotating stream sprinkler of the type having a rotatable deflector with spiral vanes engaged by one or more water jets for imparting a rotary drive torque to the deflector, which converts the jets into a plurality of relatively small outwardly projected water streams swept over the surrounding terrain to irrigate adjacent vegetation. The sprinkler includes a speed governor having a turbine driven orbiter with first and second gears meshed respectively with a stator gear and a driven gear having different numbers of gear teeth, wherein the driven gear is carried by the deflector for rotation therewith. The turbine rotatably drives the orbiter on an eccentric axis, to react against the stator gear and thereby rotatably drive the driven gear and deflector with a substantial speed reduction. The speed governor thus regulates deflector speed for slowly sweeping the projected water streams over the adjacent landscape.