Odontocetes first appear in the fossil record during the early Oligocene and soon after show a near-worldwide distribution. However, little is still known of their early diversity, especially in the North Pacific Region. A new taxon of stem odontocete with heterodont dentition, Olympicetus avitus, gen. et sp. nov., is described herein from the upper Oligocene Pysht Formation in Washington State, U.S.A. The material consists of two partial skulls belonging to a juvenile and a neonate and includes part of the dentition and a tympanic bulla. Olympicetus shares many similarities with the Oligocene odontocetes Simocetus rayi and Ashleycetus planicapitis but displays a combination of characters that distinguish it from these and other coeval taxa. A phylogenetic analysis shows Olympicetus in a polytomy with Xenorophidae, a clade of early high-frequency-hearing odontocetes mainly known from the western Atlantic. Furthermore, Olympicetus displays characters that can be considered as structural proxies for echolocation, such as having premaxillary sac fossae and the maxilla expanded over the frontal. In addition to Olympicetus, other marine tetrapods from the Pysht Formation include marine birds, desmostylians, pinnipeds, and early mysticetes, giving us a unique insight into late Oligocene marine tetrapod faunas of the North Pacific.