Abstract: The tightener is designed for use with belts driving oil well pumps but of course may be used in other environments. It consists of a belt idler pulley journalled for rotation upon the distal end of a mounting arm which in turn is secured for selective rotation by the other end thereof to a mounting bracket. A spring loaded actuating arm rotates the mounting arm through an arc from a belt tensioned position to a belt untensioned position and vice versa and the actuating arm may be selectively held in the belt tensioned position. An adjustment is provided between the actuating arm and the mounting arm to preset the angle therebetween and thus the relationship of the pulley with the belt. The inner end of the mounting arm and upper end of the actuating arm are mounted on a center line between the upper and lower runs of the belt so that the device can be adjusted to engage the outer surface of either of the belt runs depending upon drive direction.