Abstract: The chain saw tensioner of the present invention provides a sheath anchored at one end to the housing of the chain saw which sheath receives a tensioning shaft slidably therein. The tensioning shaft is interconnected with the housing by a tension screw and is slidably adjustable with respect to the sheath by actuating the tensioning screw. The tensioning shaft and the sheath have matching and aligned slots that cooperate to receive a sliding bolt therethrough. The sliding bolt is interconnected with the blade of the saw and, by actuating the tensioning screw, it may be moved forward with respect to the housing of the saw to move the saw blade forward to tension the chain. Included within the tensioning shaft is a spring interconnected with a bolt rest for receiving and supporting the sliding bolt. The spring provides a bias which acts on the bolt to bias the bolt, and accordingly the saw blade, forward with respect to the housing to tension the chain.