The Yellow-spotted Mountain Stream Keelback Opisthotropis maculosa was originally described based on a single male specimen from northeastern Thailand. Recently, based on morphological data, new records of this species were published initially from southern China and subsequently from northern Vietnam. In this study, we provide the first molecular comparisons between the holotype and other populations in China and Vietnam using the mitochondrial cytochrome b gene and use an integrative taxonomic approach to show that the population from Vietnam represents a distinct taxon. Opisthotropis haihaensis sp. nov. is characterized by a combination of the following characters: internasal not in contact with loreal; prefrontal not touching supraocular; frontal touching preocular; one preocular; one postocular; one anterior temporal; one posterior temporal; eight supralabials, fourth and fifth in contact with eye; 24 maxillary teeth; anterior pair of chin shields longer than posterior pair; 169 ventrals + 2 preventrals); 79 subcaudals, paired; 15 dorsal scale rows at neck, at midbody and before vent; body and tail scales smooth; chin shields yellow with brownish black mottling; body and tail dorsum dark with each a light spot per scale. Phylogenetically, the new species is supported as the sister taxon to "O. maculosa" from China (but separated by approximately 10% uncorrected pairwise sequence divergence) and is distantly related to O. maculosa sensu stricto from Thailand, warranting a taxonomic revision of the maculosa-like species. According to our results, O. maculosa should be delisted from the herpetofauna of Vietnam, which currently consists of nine Opisthotropis species. Five species, O. cucae, O. daovantieni, O. haihaensis, O. tamdaoensis, and O. voquyi, are endemic to the country.
Keywords: Reptilia, Opisthotropis maculosa, Opisthotropis haihaensis sp. nov., morphology, phylogeny, taxonomy.