Abstract: A tensioner is provided for a power transmission belt comprising at least one return spring associated with the shaft of a tensioning roller with which the belt cooperates for controlling the displacement of said roller under the action of said spring, and two variable volume chambers each defined by an elastomer sheath, filled with a liquid and communicating with each other at one of their ends through a valve device through which the liquid flows into or out of said chambers and which introduces a damping effect of the vibrations likely to occur in the belt, wherein said sheath and a first spring portion associated with one of the chambers are fast with each other, by enveloping said first spring portion in said sheath, and said sheath and said second spring portion associated with the other chamber are independent of each other, said second spring portion surrounding the sheath and working under extension.