Pterophocaena nishinoi, gen. et sp. nov. (Phocoenidae), from the upper Miocene Wakkanai Formation (9.2–9.3 Ma) in Hokkaido, northern Japan, is described. This is the oldest fossil phocoenid in the western North Pacific, thus extending the fossil record of Phocoenidae an additional approximately 4 million years in the region compared with the previous record. The holotype, composed of a partial skull, with right periotic and tympanic bulla and a postcranial skeleton, has the following apomorphic characters: absence of the premaxillary eminence; posterior half of the hamular process of the pterygoids not separated by palatines; prominent dorsolaterally projecting premaxilla in the facial area; supraorbital process of the maxilla steeply sloping ventrolaterally; posteriorly protruding temporal crest; anteroventrally narrowed and inverted teardrop-shaped temporal fossa; and several short ridges on the tympanosquamosal recess of the squamosal. A comprehensive cladistic analysis (66 ingroup taxa and 278 morphological characters) indicates that Pterophocaena is the most basal phocoenid yet discovered, and that Phocoenidae and Delphinidae are closely related. The discovery of Pterophocaena leads to a review of previous hypotheses for the origin of Phocoenidae, and indicates that phocoenids had already diversified in the Pacific Ocean by late Miocene.