We document the naturalization of Alnus nepalensis D. Don (Betulaceae), native to southern Himalaya from Pakistan to southwest China, has been found at middle altitude in central Taiwan. A taxonomic treatment, line drawings, color photographs, and distributions are provided to aid in identification of this alien tree. A detailed comparison table between A. nepalenssi D. Don and A. formosana (Burkill ex Forbes & Hemsl.) Makino are also provided.