The invention relates to a hydraulic tensioner for tensioning a traction mechanism of an internal combustion engine, having a housing in which are arranged a high-pressure space and a low-pressure space which are filled with oil and can be connected by means of a ball valve and a throttle gap. A tensioner (1, 1′) which maintains the oil quality of its first filling for the longest possible time is provided in that the low-pressure space (12) is connected by means of a leakage oil line (15) to the ball valve (7) and by means of an external oil line (16) to an oil source, and in that another ball valve (17) is situated in the external oil line (16), which other ball valve (17) can be closed off in the direction of the oil source.