Astomum japonicum G. Roth is a cleistocarpous moss species but its taxonomic placement remains uncertain because the type material has not been re-examined since being described in 1911. We have examined type specimens and selected the specimen in PC as the lectotype. Based on a morphological study of the type material, we also propose the transfer of the species to the genus Weissia Hedw.: W. japonica (G. Roth) Y. Inoue & H. Tsubota comb. nov. Taxonomic notes on W. japonica and a key to the cleistocarpous species of Weissia in Japan are also provided.