Desmatophocidae (Mammalia, Carnivora) represents the first of the four major pinniped clades to appear in the fossil record. However, the majority of its known diversity consists of derived species and little is known about the nascence of this early pinniped lineage. Here we report the discovery of Eodesmus condoni, gen. et sp. nov., represented by a nearly complete cranium from the Burdigalian Iron Mountain Bed of the Astoria Formation from the central coast of Oregon, U.S.A. Notably, this specimen possesses nasolabialis fossae, a plesiomorphic trait shared with stem pinnipedimorphs and the basal phocid Devinophoca sp., but not with any other desmatophocid. Phylogenetic analysis (98 characters, 27 taxa) resolved Eodesmus condoni as the most basal desmatophocid yet described. Poor support for the monophyly of Otarioidea, Odobenidae, and Phocoidea demonstrate that high-level relationships within Pinnipedia have yet to be conclusively resolved. The identification of another desmatophocid species at the already pinniped-rich Iron Mountain Bed suggests that the approximately synchronous mid-Miocene Climatic Optimum played a role in this increase in pinnipedimorph species diversity.