Disclosed is a belt tensioner having a single-piece plastic body, having a fastening section, that has an adjustment mechanism via which the plastic body can be adjustably fastened, to the carrier; a pressing section configured to press against the drive belt in order to tension the drive belt; and a spring joint device which connects the fastening section and the pressing section to one another, and via which the pressing section is displaceably supported at the fastening section along the deflection direction so that, when the pressing section presses against the drive belt, the amount of the displacement of the pressing section along the deflection direction is dependent on the respective set drive belt tension, wherein the belt tensioner has a setting aid that indicates when the predefined drive belt tension is set by the belt tensioner.