A hydraulic locking and loosening device (40) is set on the neck of a rolling-mill roll or back-up roll with a conical part (3) and a cylindrical part (4), and comprising an oil-film bearing (7, 8), a spacer ring (16), a thrust bearing (15), a thrust ring (17), and a threaded lock ring (18). The locking device (30) comprises a body (21) inserted at the centre in a ram (22) to form a hydraulic actuator with two annular chambers (23), longitudinal arms (31), on each of which is provided a C-shaped notch (32), and cover (24) for closing the hydraulic chambers (23). A variant of the hydraulic locking and loosening device (40) is provided with a bayonet device for its engaging on the roll end.