Haborophocoena toyoshimai (family Phocoenidae), from the early Pliocene Mochikubetsu Formation in northwestern Hokkaido, Japan, is a new genus and species of porpoise. The holotype is an incomplete skull, tympanoperiotic, mandible, and several teeth. Its features include: a robust and asymmetrical skull; a premaxilla with a less atrophied posterior extremity; a low but wide premaxillary eminence; a conspicuous knob-like frontal boss; narrow palatines posteriorly separating the pterygoids; a thin lateral lamina of the palatine that extends posteriorly; well-excavated air sinus fossae on the basicranium; a relatively small temporal fossa; an abruptly sloping supraoccipital shield; a periotic with massive anterior process; and tiny teeth with blunt tips. Some features of living phocoenids may be paedomorphic; examples include the relatively short rostrum, the rounded cranium, and the small body size. “Precocious” reproduction and a short interval of the reproductive cycle, compared to other delphinoid dolphins, suggest that paedomorphic characters of living phocoenids are progenetic. Progenetic evolution possibly occurred in harsh environments caused by glacial oscillations of the Pleistocene; molecular divergence times for porpoises are compatible with this idea.