Abstract: Described is a device for constraining the opening of doors or windows comprising a guide body associated with a perimeter profile of a movable leaf and equipped with a slot; an operating arm articulated, at a first end, to a fixed frame, and a second end slidably connectable, by a pin, inside the slot of the guide body so as to allow a sliding stroke of the pin along the guide slot to define an open configuration with a limited angle of the leaf; the guide body is provided with two lateral openings facing each other made on the walls defining the slot to allow a free passage, through one of the openings, of the pin of the arm housed in the slot when the leaf is in a free open configuration; a locking lever or latch articulated to a base of the guide body and configured for locking the pin of the arm when it is positioned at an end of the slot of the guide body in the open configuration with a limited angle of the leaf; an operating rod slidably inserted along the body and having a pair of protruding walls co