New tarsals (astragalus and entocuneiform) of fossil pinnipeds from the lower Miocene (Burdigalian) Oi Formation in Mie Prefecture, Japan, belonging to early pinnipedimorphs (i.e., enaliarctines or basal odobenids) and cf. Odobenidae gen. et sp. indet. were discovered. A comparison of early pinnipedimorph astragali conclusively indicated that at least three species were represented in the western North Pacific during the early Miocene (Burdigalian). Early pinnipedimorphs clearly showed considerable diversification in the eastern North Pacific during the early Miocene, and our study demonstrates that the early pinnipedimorphs in the western North Pacific also started to diversify around the same time.