Genus Neosinocythere is described from the upper Miocene Itahana Formation in Central Japan, documenting the first record from Japan. The Neosinocythere species were discovered in gravity-flow sediments that were deposited below a depth of 84.9 m with a paleo-summer bottom temperature of 23.7°C and a paleo-winter bottom temperature of 14.1°C, based on a modern analog technique using the ostracode assemblage. Paleobiogeographical and paleoenvironmental analyses indicate that Neosinocythere appeared around the southeastern coast of the Eurasian continent, extended distribution in the northern coast of the Eurasian continent before migrating southward along the coast of the Eurasian continent as global temperatures decreased. The high-diversity area shifted from high latitudes to low latitudes.