The present invention provides a hydraulic tensioner with a ratchet mechanism simple in structure. The hydraulic tensioner (1) includes a hollow plunger (3) that is slidably received in a bore (2a) of the housing (2) that has an inner space (3a) defining a fluid chamber (20) with the bore (2a) and a rack member (7) provided slidably in axial direction in a groove (3b) on an outer circumference of the plunger (3). The hydraulic tensioner (1) further includes a spring (4) biasing the plunger (3) in a protruding direction, a ratchet member (8) that that is provided slidably in a ratchet hole (2b) extending in an inclined direction intersecting an axis (L) of the plunger (3) and that has a cylindrical head portion (88) adapted to engage rack teeth (7a) of the rack member (7), permitting travel of the plunger (3) in a protruding direction but preventing retraction of the plunger (3) in a backward direction, and a coil spring (85) that biases the ratchet member (8) in an engaging direction of the rack teeth (7a) with an outer circumferential edge portion (80a) of the distal end of the head portion (80).