A hydraulic tensioner for a traction drive is provided having a spring-loaded piston (8) guided for longitudinal motion in a cylinder (6), in which tensioner, a piston motion causes an exchange of volume between a high pressure chamber (9) arranged in the cylinder (8) and a hollow space (4) which is partly filled with hydraulic medium, and, depending on a direction of motion of the piston (8), said exchange of volume can take place through a one-way valve (15), the tensioner further comprising a bellows seal (18) which is arranged between the piston (8) or the piston housing (10) at one end, and the outer housing (2) at a second end and follows the longitudinal motion of the piston (8) while bearing under pre-stress against the outer housing (2) and against the radially inward stepped piston housing (10). The bellows seal (18) of the invention is radially pre-stressed towards the piston housing (10) and/or the outer housing (2) so that, on occurrence of an overpressure in the hollow space (4), an automatic reduction of pressure of the air enclosed in the hollow space (4) can take place.