A hydraulic tensioning device (1) for a flexible drive element, particularly for a chain or a belt, is provided, and includes a housing (2) that has a cylindrical inner surface (3) at least in some regions, a piston (4) is arranged in the housing (2) in an axially displaceable manner, and an inner chamber (5) is formed by the housing (2) and the piston (4) for receiving hydraulic fluid. At least one vent hole (6) is integrated in the housing (2) and/or in the piston (4) is in contact with the surroundings (U). In order to provide a reliably ventilated system that has a simple design and that can be implemented in a cost-effective manner, a sealing element (7) is arranged in a region of the vent hole (6), and the sealing element is permeable to gas but not to hydraulic fluid.