Appliance for locking wheel rims includes: a supporting structure associable with a rotation device of a repair workshop machine; at least two locking jaws for locking a wheel rim, which are associated with the supporting structure and move along a direction of reciprocal moving close/away between a position of maximum opening, and a position of minimum opening; a movement arrangement associated with the jaws for moving them between the positions of maximum and minimum opening; an adjustment device placed between the movement arrangement and at least one of the jaws, for adjusting the position of maximum and/or minimum opening and including a substantially elongated adjustment element associated with the movement arrangement, revolving around its own longitudinal axis and having a threaded portion, and a mobile element associated integral with at least one of the jaws and having a threaded hole for engagement with the threaded portion of the adjustment element.