A new heteromorph ammonoid Sormaites teshioensis gen. et sp. nov. (Diplomoceratidae) is described from the upper Turonian (Upper Cretaceous) in the Nakagawa area, Hokkaido, northern Japan. Its near planispiral whorls consist of crioceratoid early whorls followed by straight shafts with U-shaped whorls connecting the successive shafts. The shell surface is ornamented with simple, straight, sharp tipped ribs throughout ontogeny, but infrequent flared ribs and constrictions occur on later whorls. Excluding its earliest whorls, its coiling and ornamentation are very similar to Scalarites mihoensis and Sc. densicostatus from the Turonian to Coniacian in Hokkaido and Sakhalin, suggesting that So. teshioensis was probably derived from one of these taxa in the Northwest Pacific during middle to late Turonian time.