The invention relates to a hydrostatic actuator, having a master cylinder including a housing and a piston, which is axially mountable in the housing and which pressurizes a pressure chamber filled with pressure medium, a planetary rolling-contact gear system that converts a rotary drive into an axial motion and that has a sleeve, a gear-driven spindle, and planetary rolling elements that roll therebetween, and an electric motor that drives the planetary rolling-contact gear system and that has a stator rigidly connected to a housing and a rotor that can be rotated relative to the stator. In order to optimize the installation space, the pressure chamber is annular and the planetary rolling-contact gear system is arranged radially within the pressure chamber.