Adjustment device for the locking jack 9 of a molding press comprising, on a base 1, a mold 4 held by a fixed plate 2 and a movable plate 3 controlled by a rapid movement mechanism 10-13, itself connected to a fixed plate 5 via a movable plate 8 subjected to the action of the locking jack. The adjustment device includes a stem 17 which frictionally slides in relation to the base 1 and which is fitted with arms 18, 19 situated between two control contactors 15 and 16 of the locking jack and connected to the movable plate 8. The device provides automatic adjustment of the control of the locking jack 9 in accordance with the thickness of the mold.