Small-sized ammonites assigned to the genus Tetragonites, whose ventral apertural margin projects forward in a broad convex arch, include two species, i.e., T. minimus and T. nanus sp. nov. These particular taxa occur only in the Upper Cretaceous (Turonian to Campanian) of Hokkaido, northern Japan. Tetragonites minimus occurs in the lower Turonian to the Santonian, and T. nanus, which occurs in the Coniacian to the middle Campanian, tends to be smaller in adult shell size than T. minimus. There is no significant bimodal distribution in the adult shell size of either species. The Tetragonites group characterized by a ventral projection on the apertural margin flourished mainly in the Tethyan realm during the late Aptian to middle Cenomanian time interval, but then the group mostly disappeared from the Tethyan realm, leaving only the Hokkaido occurrence of the present study. A working hypothesis can be formulated that Tethyan species such as T. spathi migrated to the Northwest Pacific region near the Cenomanian–Turonian boundary and gave rise to Turonian T. minumus and succeeding T. nanus during the Coniacian or possibly earlier. Tetragonites nanus is one of the smallest mature ammonoids known thus far.