An ultrasonic joining method for connecting paper material (5), in particular paper, board or paperboard, comprising the following steps: introducing the paper material (5) into a gap (4) between a sonotrode (2) and an anvil (3), causing the sonotrode (2) to oscillate ultrasonically for the ultrasonic welding of the paper material (5), the paper material (5) being moistened with demineralized water (9), preferably on inner sides of joining points, before and/or during the ultrasonic welding.