In a chain tightener with a dampening piston guided in a housing and axially acted upon by a spring in the chain tightening direction wherein the dampening piston is retainable by a mounting ring within the housing in a blocking position which is released through axial pressure in the direction opposite to the chain tightening direction, the inprovement comprising the mounting ring (30) snaps into a groove (29) which is located at the forward end of the dampening piston (3) and is provided with a bevelled surface (28) and an adjacent latch surface (27), and that the mounting ring (30) is shiftable onto the latch surface (27) via the bevelled surface (28) with a tool insertable in a gap (31) between the housing (1) and the dampening piston (3) wherein the mounting ring (30) engages in a notch (13) of the housing (1) thereby blocking a movement of the dampening piston (3) relative to the housing (1).