An ultrasonic joining method for bonding paper material (5), in particular paper, paperboard or cardboard, comprising the following steps: introducing the paper material (5) into a gap (4) between an ultrasonic horn (2) and an anvil (3), applying a joining force in a joining force direction (6) to the paper material (5) in the gap (4), and setting the ultrasonic horn (2) into an ultrasonic vibration in order to join the paper material (5), characterized in that an ultrasonic frictional welding operation is carried out, in which the joining force direction (6) is not parallel to the direction of vibration (7) of the ultrasonic vibration.