An assemblage of eleven species of Early Eocene marine intertidal and sublittoral ostracods has been found in the Crescent Formation in the Black Hills, Thurston County, Washington State, USA. The ostracods are subtropical and tropical taxa, such as Bairdoppilata, Cytherella and Neomonoceratina, which are found south of Washington today. Based on the geological age of the formation, the fauna corresponded to the Early Eocene Climatic Optimum. Five new species are recognized: Acanthocythereis olympiana sp. nov., Ambostracon irizukii sp. nov., Bairdoppilata crescentana sp. nov., Coquimba washingtonensis sp. nov. and Cytherelloidea squiresi sp. nov. This new assemblage from the Crescent Formation is the northernmost record of Early Eocene ostracods for the entire Pacific Basin.