A method for joining a metal material element to a plastic material element, in particular a composite material including a plastic matrix reinforced with fibers for use in the construction of motor-vehicle components. The method involves providing one or more slots in a portion of the plastic material element. For each slot, a corresponding tab is provided in the metal material element, having a shorter width and length than a width and length of the slot. The metal material and plastic material elements are arranged in a position of mutual coupling where each tab is inserted through the corresponding slot and has an end portion protruding beyond said portion of the plastic material element. A laser beam is directed above the protruding end portion, so as to locally melt the metal material of each tab and create an enlarged head on each tab that is welded above the plastic material element.