Seven species of Gastropoda and 21 species of Bivalvia were discriminated from the Miocene Ichibu Formation on Nishinoshima Island, Oki Islands, Southwest Japan. The co-occurrence of Nipponopecten akihoensis (Saga), Chlamys (Nomurachlamys) kaneharai (Yokoyama), and Miyagipecten matsumoriensis indicates a latest middle—earliest late Miocene age (ca. 12.5–11.5 Ma) for the formation. The Ichibu molluscan fauna commonly includes eurythermal and North Pacific elements, whereas only two warm-water species were recognized. This fact shows that the shallow sea around the Oki Islands was under a temperate marine climate weakly influenced by the Paleo-Tsushima Current in latest middle—earliest late Miocene time. Taxonomically described and/or discussed species include Costanuculana godaigoi Matsubara, sp. nov. and Yabepecten okiensis Matsubara, sp. nov.