Abstract: A wild population of the globally restricted and highly threatened freshwater fish Tanichthys albonubes Lin was recently discovered on Hainan Island, China. Prior to the present study, native populations were only known from isolated pockets in Guangdong Province of China and Quang Ninh Province of Vietnam; the Hainan record thus represents the first insular and southernmost distribution of this little-known species. Thus far it is known only from a single site in Hainan, despite repeated surveys of similar habitats throughout the island. The Hainan population occurs in a clean, sluggish coastal stream with abundant hydrophytes, and is sympatric with 20 fish species. Due to the unique geographic distribution of the Hainan population, and the conservation significance of the species, studies of its phylogenetic relationships with conspecifics populations elsewhere are underway. The exact locality of the newly discovered Hainan population cannot be revealed, but relevant government authorities have been notified and we are working to protect the site.