The spioniform polychaetes including Spionidae, Magelonidae and Owenidae herein described were mainly collected by the author from various localities in Japan and the adjacent regions, ranging from Hokkaido to the Korean Archipelago, and four species among them were obtained by Dr. M. Veno, Mr. F. Hiro and the authorities of the Onagawa Oceanographic Station and the Hokkaido Fishery Station. So far as I am aware, only seven species belonging to the Spionidae and Owenidae have formerly been reported from Japan, by Moore (1907), McIntosh (1915), Söderström (1920) and the author (1935). No representative of the Magelonidae has yet been recorded from Japanese waters. I have in this paper 16 more species to be added to the Japanese fauna. Among them are five species, including one variety, which appear to be new to science. The following is a list of the spioniform polychaetes, belonging to the families above mentioned, reported hitherto from Japanese waters. The species newly reported have been marked with an asterisk. Those belonging to the Chaetopteridae are excluded, because they were already described in the separate paper.