The invention relates to a tensioning device (8) for an endless drive element, in particular a timing chain (5) of an internal combustion engine, comprising a housing (10), a tensioning piston (11) which can be movably guided in the housing (10), and a pressure chamber (19) for a hydraulic fluid, formed between the housing (10) and the tensioning piston (11), wherein the tensioning piston (11) has a ventilation opening (15) and a ventilation channel (23, 16.1, 16.2, 16.3, 22.1, 22.2, 22.3) is provided which is disposed between the pressure chamber (19) and the ventilation opening (15) and which is formed by the assembly of at least two ventilation elements. In one ventilation element, in the surface (E) which contacts the second ventilation element, sub-regions of the ventilation channel which are separated from one another are formed by indented groove sections (16.1, 16.2, 16.3, 22.1, 22.2, 22.3), which are connected to one another by transitional grooves (22.1, 22.2, 22.3) formed in the second ventilation element in the surface which contacts the first ventilation element in such a way that a main part of the flow cross-section of the ventilation channel is disposed with some sections on one side and some sections on the other side of a plane spanned by the contact surfaces. The invention further relates to a corresponding traction drive (1) having such a tensioning device (8).