Adjustable Riser is provided for coupling an irrigation sprinkler to a water supply line. It is adapted for quick and easy longitudinal adjustment to variably select the elevational position of the sprinkler relative to ground level. It comprises a pair of telescopically interfitting inner and outer riser tubes coupled respectively to the sprinkler and to the water supply line. The inner riser tube has a keyway formed on the exterior surface and outer riser tube carries at least one key protruding into and engaged with the keyway.