The subject-matter of the invention is a hydraulic seal mounted in a radial groove and having a preloaded sealing ring, primarily intended for piston and piston-rod seals. The essential feature of the seal designed in accordance with the invention is that it has a back-up ring (7) resting under preload, by its pressure surface (8), on the side face (6) of the sealing ring (4). In the space bounded by the sealing ring and the tapered disc (12) of the back-up ring there is furthermore a clamping ring in contact with the sealing ring and the tapered disc (Figure 1). <IMAGE>
