The method and device for sealing a narrow tape to a strip of heatsealable material in cigarette packing machines, which produce packets of cigarettes protected by an external wrapper made of plastic which can be torn using the said tear tape; to avoid a thermal degradation in the sections of heatsealable strip adjacent to the sealing zone, the present invention envisages sealing the narrow tape to the strip by passing them through a sealing unit, in contact with one another, and joining them with an ultrasound sealer which directs the ultrasound to at least a zone in which the narrow tape and strip overlap; the intensity of the heatsealing action of the ultrasound is controlled and adjusted by elements which detect the feed speed of the narrow tape and strip.