Abstract: A tensioning device, in particular for a highly dynamic endless drive, such as a chain drive or a belt drive for an internal combustion engine, comprises a tensioner housing, a tensioning piston displaceably arranged in the tensioner housing, a pressure chamber formed between the tensioner housing and the tensioning piston and a pressure medium inlet leading into the pressure chamber and including a non-return valve. In order to meet also new demands, in the case of which oil pumps with having a low feed pressure are used, the present device discloses that a valve body of the non-return valve is defined by a leaf spring diaphragm. The device additionally relates to a non-return valve as well as an endless drive.