Chelonocytherois omutai gen. et sp. nov., is found on the back of female loggerhead sea turtles on the nesting beach of the Yakushima Island, Kagoshima Prefecture, Japan. The present study provides the first species-level identification and description of Ostracoda as sea turtle epibionts. The new genus and species, which belongs to the family Paradoxostomatidae Brady and Norman, 1889, can clearly be distinguished from other genera by the morphology of the carapace and the appendages. Especially, the quite large sieve structure on the pore system of the carapace is characteristic to this genus. Although a similar structure is also found in the commensal ostracod genus Redekea de Vos, 1953, the new genus has distinctive mandibular and maxillular morphologies. From examination of the appendages, C. omutai gen. et sp. nov. seems to be an epibiotic species associated with sea turtles, but not a strictly commensal species. Additionally, the nearly complete 18S rRNA gene sequence is determined for C. omutai gen. et sp. nov. for future molecular phylogenetic studies. Since the sea turtles migrate across the oceans in their ontogenetic stages, they may play a role as a long-distance dispersal vector for C. omutai gen. et sp. nov.