The Early Pennsylvanian Nagaiwa Formation contains fossils such as corals, fusulinids, and ostracodes, and its age and depositional environments have been determined by fusulinids and sedimentology. In this study, I describe the ostracode assemblages from the Nagaiwa Formation. Moreover, I provide a reconstruction of the paleogeography of northeastern Japan during the Early Pennsylvanian by comparing this ostracode assemblage with assemblages from other regions during the same period. Thirty ostracode species, including 12 genera, have been identified, most of which are endemic species and 10 of which are new: Jordanites michinokuensis n. sp., Thuringobolbina ikeyai n. sp., Aechmina iwatensis n. sp., Pseudobythocypris asiatica n. sp., P. zipangu n. sp., P. siveteri n. sp., Platyrhomboides tohokuensis n. sp., P. japonica n. sp., Healdia ofunatensis n. sp., and H. rikutyuensis n. sp. Two of these species are also found in central Japan. The ostracodes from the Nagaiwa Formation are unique when compared with any other similarly aged assemblages.