A locking mechanism (28) is disclosed for use on the legs (14) of a jack-up offshore platform (10). Each chord (18) of the legs (14) includes a rack (22) cooperating with pinions (26) used in raising and lowering the legs. The locking mechanism (28) includes a rigid frame attached to the hull (12) of the platform (10) and housing (36) and a plurality of vertically aligned toothed chock segments (30, 32, 38) to engage the teeth (24) of the rack (22) on a leg (14). The chock segments have upper and lower inclined surfaces cooperating with upper and lower movable wedges (48, 52) for supporting the chock segments (30, 32, 38). Subsequent engagement and retention of the supporting wedges (48, 52) securely locks the chock segments (30, 32, 38) into mating engagement with the rack teeth (24), to thereby lock the platform legs (14) in place.