A mounting jig for a vehicle door, the jig includes a first support having means for adjustably fastening it to a vehicle body in the vicinity of the door opening; a second support having fastening means for fastening to the vehicle door; connection arms for interconnecting the first and second supports, which arms are hinged to each of said supports, and their trace, in a plane perpendicular to the plane of the door opening, co-operates with the trace, in the same plane, of said supports, to form a deformable quadrilateral; a weight arm, mounted to move at least in translation along a vertical axis in guide means carried by the stationary support, between a low first position and at least one second position that is higher than said low position; at least one operating member for operating an indexing member for indexing the position of the second support relative to the first support, the operating member being connected to the weight shaft; and a raising lever, for acting against the effect of gravity, to raise the weight shaft from the first position towards the second position.