Invertebrate fossils described from ancient hydrocarbon seep deposits represent diverse groups, e.g., brachiopods, mollusks, decapod crustaceans, worm tubes, and rare echinoderms, but the fossil record of ostracodes from hydrocarbon seep deposits is still very limited, making their ecology and evolutionary history still little known. We found fossil ostracodes in eight Eocene to Oligocene hydrocarbon seep deposits in the Humptulips, Lincoln Creek, Makah, and Pysht formations in western Washington State, USA. They represent eleven taxa belonging to genera found in a wide range of shelf to slope habitats: Acanthocythereis, Loxoconcha?, Cytherella, Cytheropteron, Macropyxis?, Krithe, Paracosta, Pontocythere?, Propontocypris, Palmoconcha, and Neonesidea? Acanthocythereis acroreticulata from one late Oligocene seep deposit in the Lincoln Creek Formation is the oldest and northernmost record for this species. The hydrocarbon seep ostracode faunas from Washington appear to be benthos-dominated, showing the same ecological structure and pattern of phylogenetic relatedness as ostracodes from Miocene and Quaternary seep sediments from Italy and off Ireland. We suggest that the benthos-dominated structure has been stable for ostracodes in hydrocarbon seep environments and/or has a higher preservation potential than the nektobenthos-dominated structure.