Approximately half of the threatened Japanese tidal-flat molluscan species live in Nansei-shoto Islets, and 84% of them (71 species) distribute in Okinawa-jima Island. Twenty-four out of 71 species are endemic or limited to Okinawa-jima. Therefore, Okinawa-jima is one of the most important habitats in Nansei-shoto for those molluscs. A vast tidal flat with an area of 1,962 ha used to exist in Okinawa-jima in 1945. However, 75% of the area has been lost by 2012 due to reclamation. After endangered species in Okinawa-jima was evaluated quantitatively, all 24 species were considered the critically endangered species. As for the tidal-flat molluscs of southern part of Okinawa-jima Island, species diversity was already decreased by coastal development for economic activities. The first extinct species of Japanese tidal-flat molluscs was recognized in Okinawa-jima Island. Natural bay areas still remain in Oura bay and Haneji naikai inland sea, northern part of Okinawa-jima. We must protect these bay areas where the threatened tidal-flat molluscs are able to inhabit.