Abstract: A hydraulically or pneumatically controlled device for locking a piston in he bore of its cylinder at any point of its stroke, comprises, mounted on the piston, fluid-tight seals, a series of wedges, and annular jacks incorporated in the piston. The wedges are arranged to move in translation parallel to the axis of the cylinder under the effect of the annular jacks so as to absorb the increase in the radial plays between the piston and the bore of the cylinder on its pressurization. This peripheral extension which is produced between the bottom of the cylinder on its intake side and the fluid-tight end seal of the piston spaced furthest from it, is followed, after the release of the lateral walls of the cylinder, by a retraction of the lateral walls of the cylinder acting on the outer surface of the wedges. The latter exert a grip on the piston thus effecting positive locking by binding of the cylinder and of the piston.