Many species of flightless rail are endemic to small islands and endangered, or have become extinct during historical times. However, our understanding of their basic ecological knowledge remains limited. The Okinawa Rail Hypotaenidia okinawae is a flightless species of rail endemic to Okinawajima Island in the Ryukyu Archipelago, Japan. In this study we have quantified the dietary components of the Okinawa Rail using gizzard contents classified by the frequency of occurrence and wet weight. The Okinawa Rail feeds on a wide variety of animals and plants throughout the year, but the frequency of occurrence and wet weight of animal matter significantly exceeds those of plant matter. The number of items present in the gizzard did not differ among seasons, ages and between sexes. Grit was present in almost all individuals and the wet weight did not differ between sexes or among age groups. Although the diet was observed to be diverse, the primary dietary component of both sexes, all ages, and throughout the year appears to be land snails, indicating their great significance as a source of food.