A cartridge tensioner (10) for drive transmission belts and chains comprises a cylinder (12) with a cylinder bore, a piston (14) slidably received in the cylinder bore and provided with a piston bore (40), a vent device (16) housed in the piston bore and comprising an end part and a stem integral with each other, a pressure spring (18) acting between a shoulder of the cylinder and the vent device, and a retaining device (20), accessible from the outside of the cylinder. The retaining device, in a position engaged with the stem of the vent device, retains the vent device preventing the action of the spring on the piston, and, in a position of disengagement from the stem of the vent device, allows the spring to bias the device against the piston and to act consequently on the piston.