The Family Kentriodontidae is a diverse and abundant group of odontocetes from the late Oligocene and Miocene around the globe. Despite nearly a century of research on the group, the relationships across the family have remained poorly understood. Several recent studies have suggested that the family is polyphyletic and in need of major taxonomic revision. A new fossil stem delphinidan from the early Miocene of Washington State is described as Wimahl chinookensis, gen. et sp. nov., and is assigned to a revised Kentriodontidae. Wimahl chinookensis represents one of the oldest kentriodontids and is the northernmost kentriodontid described from the Pacific Ocean, extending their range in the eastern Pacific from central California up to Washington State. Its phylogenetic position relative to other stem delphinidans suggests that it is sister to Kampholophos serrulus from the middle Miocene Monterey Formation of California. Additional phylogenetic results guide the revision of stem delphinidans and new diagnosis and delimitation of the Family Kentriodontidae.