Cythere ikeyanoriyukii n. sp., an extant phytal ostracode genus, was obtained from the Middle Eocene McIntosh Formation in the Doty Hills, western Washington State, USA. It was associated with eleven taxa, which are extant phytal and shelfal genera such as Loxocorniculum, Xestoleberis, Ambostracon, Coquimba, and Acanthocythereis. The presence of Cythere in this assemblage is surprising and indicates that the first appearance of this genus was middle Eocene time at the latest, or at least 20 Ma earlier than previously thought. Cythere did not originate in the Pacific Ocean around Japan as previously thought, but instead must have migrated from the northeastern Pacific to the northwestern Pacific between middle Eocene and early Miocene time.