In the genus Triarthron (Coleoptera, Leiodidae, Leiodinae), only two species are known to occur in Palearctic and Nearctic regions. In this paper, a new species in Japan, Triarthron itoi Hoshina, sp. nov., is described. This brings the number of species in the genus to three. A key to the Japanese species of the tribe Sogdini is given.