Abstract. The genus Semicytherura (Ostracoda, Crustacea) is distributed widely in shallow-sea areas of the Northern Hemisphere and occurs commonly in Pliocene and Pleistocene strata along the Japan Sea coasts. Four new species -Semicytherura robustundata sp. nov., Semicytherura subslipperi sp. nov., Semicytherura leptosubundata sp. nov. and Semicytherura tanimurai sp. nov. - are described from the Early Pleistocene Omma Formation, central Japan. These species are palaeobiogeographically significant in the history of species diversity changes in Japan Sea benthic fauna during the Late Cenozoic. The geological and geographical occurrences suggest that these four species originated within the Japan Sea from the Late Pliocene, including one species that diversified by heterochronic evolution, and were endemic to the Japan Sea. They became extinct within this sea during the Early Pleistocene.