A rear diffuser on the underbody of a motor vehicle having an air duct which extends at the rear of the motor vehicle counter to the longitudinal direction of the vehicle toward the rear end of the motor vehicle. The air duct has at least one upper wall, which delimits the air duct in the upward direction. The upper wall has a first region, which is coupled in a nonadjustable manner to the motor vehicle and/or to the underbody. The upper wall has a second region, which can be moved relative to the motor vehicle and/or to the underbody. The second region is coupled in a movable manner to the first region by at least one hinge-type region.