The present paper describes a new species of Glycymeris (s.s.) from Ryukyu Islands, southwest Japan. The species has been known by Japanese malacologists under the name of Glycymeris "spurca (Reeve, 1843)". The new species, G. shutoi, however, differs from G. spurca by lacking finely striated primary ribs. Superficially, G. shutoi also resembles Glycymeris reevei (Mayer, 1868), G. fringilla (Angas, 1872), and G. amamiensis Kuroda, 1930 from various localities in the Western Pacific waters. Except for G. fringilla, G. shutoi, G. reevei and G. amamiensis are sympatric in waters around Ryukyu Islands and can be discriminated from each other by the differences of some characters, such as shell ornamentation, shape of ligamental area, and numbers of ventral crenulations. According to a biometrical analysis based on a thanatocoenosis of glycymeridid shells from Miyako Island, G. shutoi and G. reevei seems to represent two Mendelian populations respectively, because the frequency distribution concerning several quantitative characters appears to be a normal distribution in each species. This new species also differs from G. fringilla from Queensland by numbers of ligamental grooves and ventral crenulations.