A new thyasirid bivalve, Thyasira inadai n. sp., is described from the Japan Sea. This species lives in the Japan Sea and Yellow Sea and can be separated from the relatively shallower-water dwelling T. tokunagai Kuroda & Habe, 1951 by having a larger shell, a long and narrow auricle, a medial flattened area, an excavated lunule demarcated by an incision, a narrow apical angle and a larger prodissoconch. Based on the size of the prodissoconch and on molecular data, T. inadai n. sp. is related to the north Atlantic species, T. gouldii (Philippi, 1845). This phylogenic relationship is likely due to migration through the Bering Strait, which has opened since the latest Miocene.