The genus Xystodesmus Cook, 1895 (most closely related to Riukiaria Attems, 1938) is retained as a taxon of xystodesmine millipedes with a relatively small body length (25–35 mm), male gonopods more complicated than just a forceps-like conformation, and with live colouration of grey-brown tergites with red, orange, or yellow paranotal spots. Seven new species of the genus (Xystodesmus fasciatussp. nov., X. keramaesp. nov., X. kumamotoensissp. nov., X. kumeensissp. nov., X. parvussp. nov., X. rebekaesp. nov., X. sesokoensissp. nov.) are described from the islands of Kyushu, Okinawa-jima, Kume-jima, Okinoerabu-jima, Aka-jima, Amami-O-shima, and Sesoko-jima, southwestern Japan. Koreoaria Verhoeff, 1937, syn. nov. is synonymised with Xystodesmus Cook, 1895, so X. pallidus (Verhoeff, 1937), comb. nov. (ex Koreoaria pallida), and X. amoenus (Takakuwa, 1942), comb. nov. (ex K. amoena) are established. Furthermore, X. variatus (Pocock, 1895), comb. nov. (ex Fontaria variata), and X. saltuosus (Haga, 1968), comb. nov. (ex Rhysodesmus saltuosus) are re-evaluated and redescribed, based on re-examination of types and of freshly collected material (X. variatus only). All new species, as well as X. martensii (Peters, 1864), X. nikkoensis (Chamberlin & Wang, 1953), and X. variatus are illustrated with colour habitus photographs taken of live specimens, to facilitate field identification.
