A new agriochoerid, Protoreodon walshi new species, from the middle Eocene Santiago Formation, San Diego County, California, is described from the Jeff's Discovery and Rancho del Oro local faunas, considered to be late Uintan in age. It differs from most other species of Protoreodon in having an undivided P 4 para-metacone and a P 3 protocone connected to the paracone by a buccolingual crest. Associated postcranial material is abundant and well preserved. The postcranial skeleton shows several features which probably represent the primitive oreodont condition, including closely appressed metapodials, hoofed ungual phalanges, the lunar in broad contact with both the magnum and the unciform in anterior view, and having the magnum larger than the trapezoid. Two specimens of a larger morph, Protoreodon cf. walshi, show a different P 3 morphology and might represent another new species. Diversity of agriochoerid oreodonts in California is at least as high as has been reported for the Vieja Group of Texas, although not as high as in the Uinta Basin of Utah, where the genus Diplobunops is also present.