The Pleistocene sea lion Proterozetes ulysses belongs to a monophyletic group of North Pacific otariids that includes the living genera Eumetopias and Zalophus. Mandibles of Proterozetes from the Port Orford Formation of Oregon are described and found to be intermediate in morphology and size to the other North Pacific otariids, most resembling Eumetopias. Among other characters, the presence of an oblong lingual prominence on the anterior lingual surface of the mandible links the taxon to Eumetopias, which commonly expresses the same feature. Uniquely among North Pacific otariids, Proterozetes possesses a p1 alveolus distinctly smaller than that of the p2. The presence of a distinct but morphologically intermediate taxon in the eastern North Pacific during the Pleistocene establishes it as a time of increased otariid diversity, in contrast to a suggested decline in other marine mammal clades. The timing of sea lion diversification along the west coast of North America appears to follow diversification in Japan, reinforcing the possibility of an eastward dispersal trend in the North Pacific after the end-Pliocene.