A very small-sized ammonoid, i.e., Tetragonites pusillus sp. nov., was newly found from the upper lower to middle Maastrichtian (Pachydiscus flexuosus Zone) in the Makarov area, southern Sakhalin, Russian Far East. The new taxon is characterized by a sub-rounded whorl section and a fairly narrow umbilicus with an oblique wall, and an aperture whose ventral margin either runs straight across the venter or exhibits a slightly concave sinus. Adult shells, 13–23 mm in diameter, exhibit no signs of dimorphism. Tetragonites pusillus sp. nov. morphologically most resembles T. superstes, which is known from the Santonian to the Campanian of South Africa and Madagascar and from the lower Maastrichtian of northeastern Mexico. It is suggested that the former species gave rise from the latter during the migration from the Gulf of Mexico to the Northwest Pacific region via the tropical eastern Pacific in the early Maastrichtian time. In the Northeastern Pacific region, T. pusillus sp. nov. could have survived at least until early late Maastrichtian time.