An integrative taxonomic analysis confirmed the new species status of a recently discovered upland population of Ansonia from Thongsong, Thongsong District, Nakhon Si Thammarat Province, Thailand from the sky island archipelago south of the Isthmus of Kra. Ansonia infernalis sp. nov. is a member of the Thai-Burmese clade within the more inclusive Thai-Malaya Peninsula clade of western and southern Thailand. It is separated from all other species of Ansonia by a unique combination of morphometric and discrete morphological and color pattern characteristics and is the sister species of a clade of eight other species found north of the Isthmus of Kra. Ansonia infernalis sp. nov. is the newest member of a long list of range-restricted endemics from the sky island archipelago of the Thai-Malay Peninsula and continues to underscore the unexplored nature of this region and its need for conservation.