A new extinct species of the genus Mauremys (Testudines: Geoemydidae) is described on the basis of three fossils from the late Pleistocene deposit of Tomori Amaga Cave on Miyakojima Island of the Miyako Island Group, Southern Ryukyus, Japan. Of these fossils, two (the anterior half of the plastron and the nuchal) were previously tentatively identified as Mauremys mutica, an extant species whose distribution is currently confined to the Yaeyama Island Group within the Ryukyus. The turtle represented by these two specimens and another, previously unreported material (left third peripheral) actually most resembles M. japonica from mainland Japan and the Northern Ryukyus, and M. yabei from the middle Pleistocene of mainland Japan. The fossil turtle however, differs from the other two in exhibiting a medial length of the entoplastron greater than the interhyoplastron in ventral view, and in having a longitudinal groove medial to the gulo—humeral sulcus on the epiplastron evident in dorsal view. The present finding strengthens endemicity of the recent terrestrial fauna of the Miyako Island Group.