Variation in morphological characters were investigated among 126 specimens from at least 67 populations covering the whole range of the large pitviper currently known as Trimeresurus sumatranus (Raffles, 1822). The results showed that two distinct taxa are involved. Herein Trimeresurus sumatranus is redefined. In order to fix the status of this species, a neotype is selected and described. Its type locality is restricted to the vicinity of Bengkulu, Bengkulu Province, Sumatra. The second taxon represents a distinct, previously unnamed species, which is described. The new species differs from Trimeresurus sumatranus by a lower number of ventrals in males (162–179 against 178–185) and females (164–171 vs. 175–191); a distinctly longer tail in males (value of the ratio tail length/total length: 0.201–0.210 vs. 0.150–0.168), the color of the tail (see the description), the color of the eyes: green in the new species, vs. dark grey in T. sumatranus, the color of the ventrals, which are green with a pale posterior suture in the new species and pale with dark posterior suture in T. sumatranus. The new species lives in higher elevations than T. sumatranus and seems to be endemic to the higher mountain ranges of western Sumatra.