An electromechanical-hydraulic piston actuator providing pressurized pressure medium for a brake system of a vehicle, including an electric motor having a stator and a rotor, a rotation-translation mechanism driven by the electric motor and which has a rotatable threaded nut and a threaded spindle prevented from rotating and displaceable in its axial direction, a piston coupled with the threaded spindle in the axial direction thereof, a hydraulic cylinder having a hydraulic chamber filled with pressure medium into which the piston is displaceable from a rear piston position towards a forward piston position to pressurize the pressure medium and/or expel it from the chamber. A hydraulic connection is connected to the chamber via which pressure medium can be expelled from the chamber. An isolation device is provided for the piston-travel-controlled isolation of the rotational coupling between the rotor of the electric motor and the threaded nut.