Abstract: A screw actuator, particularly for a brake calliper, comprising a housing (1), a screw mechanism (4) with a screw (6) and a nut (5), one of which is rotatably supported with respect to the housing (1), a motor (7) which is drivingly connected to the screw mechanism (4), a mechanical locking device comprising a pawl member (41) and a rotatable member (42) connected to a rotatable component of the motor (7) and/or the screw mechanism (4), and a mechanical or electromechanical control mechanism for displacing the pawl member (41) between a locking position and a free position. A gear reduction mechanism (20) is provided, one of the gear wheels (42) thereof carries teeth (45) for engaging with the pawl member (41).