Desmostylusis an extinct marine mammal genus that belongs to Desmostylia, a clade of extinct herbivorous mammals. While desmostylian remains are widely reported from Paleogene and Neogene marine strata of the North Pacific Rim, occurrences of the genusDesmostylusare almost entirely limited to middle Miocene strata, with only a few early Miocene records from Japan. Here we report aDesmostylustooth from the earliest Miocene (Aquitanian) Skooner Gulch Formation in northern California, USA. This specimen exhibits cuspules around the crown, a primitive trait of the subfamily Desmostylidae, as seen in more basal branching desmostylid taxa such asCornwalliusandOunalashkastylus, but with a high tooth crown and thickened enamel. The specimen is also diagnostically different from all other desmostylid genera, such asCornwallius,andOunalashklastylus. The Aquitanian age of the Skooner Gulch Formation implies that the distinctive tooth morphology ofDesmostylushas persisted, largely unchanged, for more than 15 million years and that desmostylids possibly originated in western North America.