Abstract: A device for adjusting the tension of a drive belt such as the belt used to drive an alternator or other accessory in an automotive engine. The tensioning device includes one angle bracket secured to a slotted plate having an arcuate slot formed about a pivot axis on which the accessory is angularly adjustable. An adjustment bolt secures another angle bracket to the slotted plate and is extended through the arcuate slot and threaded into the accessory. A machine screw extends between the two angle brackets and is threaded through one of them. The screw drives the brackets toward and away from one another to adjust the belt tension when the screw is turned with the adjustment bolt loosened. When the adjustment bolt is tightened, it locks the accessory in place.