The invention relates to a chain tensioner comprising a housing that forms a cylindrical receptacle, and a piston that is surrounded at least in part by the cylindrical receptacle; the inner lateral surface of the cylindrical receptacle axially guides the piston when the piston travels into and out of the housing; the chain tensioner further comprises a locking unit which limits the inward travelof the piston; said locking unit includes a stop facing the housing, a mating stop facing the piston, and a locking element that is suitable to establish a force-transmitting connection between the stop facing the housing and the mating stop facing the piston. In the position of the stop facing the housing, the housing axially forms a locking section that is located between the locking element andthe piston; said locking section includes at least one recess, thus allowing the locking element to establish the force-transmitting connection between the stop facing the housing and the mating stopfacing the piston.