A new species of the basal tylosaurine mosasaur Taniwhasaurus, is described from materials collected from upper Santonian-lower Campanian marine clastics outcropping near Mikasa City, Island of Hokkaido, Japan. The type species, Taniwhasaurus oweni Hector, 1874, was originally described from a partial frontal and right and left dentaries collected from the middle Campanian, Conway Siltstone Formation at Haumuri Bluff, New Zealand. Taniwhasaurus n. sp. shares the diagnostic features of the generic type (i.e., prefrontal forms portion of naris precluding frontal-maxillary contact), but is differentially diagnosed at the specific level by features of the teeth (number of flutes), the postorbitofrontal morphology, the contact and degree of overlap of the premaxilla and frontal, and the morphology of the jugal. Recent recharacterization and rediagnosis of Lakumasaurus antarcticus Novas et al., 2002, as Taniwhasaurus antarcticus, establishes the existence of a third species of the genus, and broadens the paleogeographic and stratigraphic distribution of these animals in Upper Cretaceous circum-Pacific basins. These three species range temporally from the upper Santonian-lower Campanian to upper Campanian, and geographically, from the northern to the southern hemisphere. Endemism of the previously recognized species of Taniwhasaurus is contradicted by recognition of the first Japanese Taniwhasaurus.