Described herein is a lock for a door of a motor vehicle provided with a closing mechanism designed to cooperate with a lock striker fixed to an upright of the door itself; the closing mechanism is provided with: a fork, which can assume an opening position, in which it enables engagement and disengagement between the lock striker and a seat thereof, and a closing position, in which it withholds the lock striker within its seat and prevents disengagement thereof; a dog coupled via snapaction with the fork for blocking it in a releasable way in the closing position; and an auxiliary lever, which can be actuated by the fork during coupling with the lock striker for exerting an action of thrust on the dog and causing it to couple with the fork itself.