Paper-based packaging materials are generally assembled using adhesives formulated with oil-based polymers. These adhesives make the recyclability of the materials more complex and may be the source of material contamination by mineral oil. In view of developing an adhesive-free process, the potential of ultrasonic compression was investigated in this study. 100% lignocellulosic papers were assembled using an ultrasonic welder dedicated to thermoplastic polymers. For papers containing lignin, the measured peeling strengths were equivalent to those achieved by hot-melt gluing, provided that the water content of papers was well adjusted. At the interface between bonded papers, the fiber network was dense and rather continuous. SEM examinations, 3D X-ray microtomography images, and temperature measurements suggested that the development of adhesion originated to a large extent from a thermoplastic welding mechanism: wood fiber polymers passed their glass transition temperatures, creeped and formed a matrix that coated fibers. Thus, ultrasonic welding appears as an efficient adhesive-free technique for assembling papers that are used in a broad range of packaging applications.