A new species of the genus Trimeresurus Lacépède is described from Kunming City, Yunnan Province, China, based on an integrative analysis of morphological and molecular data. The new species, Trimeresurus loong sp. nov. , is assigned to the subgenus Viridovipera Malhotra & Thorpe and can be distinguished from its congeners by the following combination of characters: the first supralabial completely separated from the nasal scale; a short and spinose hemipenis; moderate adult body size; dorsal scales in 19 (21 or 23)-19-15 rows; 150–151 ventral scales in males, and 157–158 in females; 67–68 subcaudals in males, and 57–64 in females; and distinct body color patterns. Molecular analyses based on the mitochondrial 16S , cyt b , and ND4 gene fragments indicate that Trimeresurus loong sp. nov. is genetically divergent from all currently recognized congeners, showing uncorrected p -distances ranging from 6.7–11.2% in cyt b and 6.1–8.3% in ND4 . The discovery of this species brings the total number of recognized Trimeresurus species to 57, including nine recorded from Yunnan Province, further emphasizing the complex topography and exceptional level of endemism.