An adjustment device for adjusting a moving portion of a motor vehicle headlight relative to a fixed portion, in particular a reflector relative to an intermediate part of the headlight, the device being of the type comprising a screw that includes a ball member received in a seat connected to one of the portions and a thread engaging in tapping connected to the other portion, wherein the ball seat and the tapping are respectively provided on two elements each including assembly means enabling it to be assembled to corresponding assembly means provided on said respective portions merely by relative displacement.