We describe a new species of the gekkonid genus Cnemaspis from Erawan National Park in Kanchanaburi Province of western Thailand. Molecular phylogenetic analyses, based on the mitochondrial NADH dehydrogenase subunit 2 gene and flanking tRNAs, revealed that Cnemaspis auriventralissp. nov. is nested within the C. siamensis group and is closely related to C. huaseesom, but has uncorrected pairwise genetic divergences of 12.12–27.92% from all other species in the C. siamensis group. The new species is also distinguished from other species in the C. siamensis group by having the combination of snout-vent length 36.7–38.6 mm in males (N = 3), 32.9–36.9 mm in females (N = 2); eight to ten supralabials; seven to nine infralabials; ventral scales smooth; six or seven precloacal pores in males; 16–17 paravertebral tubercles linearly arranged; tubercles on the lower flanks present; lateral caudal furrows present; no caudal tubercles in the lateral furrows; ventrolateral caudal tubercles present anteriorly; caudal tubercles not encircling tail; subcaudals smooth bearing a single median row of enlarged smooth scales; two postcloacal tubercles on each side; no shield-like subtibial scales; subtibial scales smooth; no enlarged submetatarsal scales; 23–27 subdigital lamellae on the fourth toe; sexually dimorphic for dorsal and ventral colour pattern; prescapular marking absent; gular marking absent; and yellow colouration in life of all ventral surfaces of head, body and tail in adult males. The new species is currently known only from upland karst habitat at its type locality.