A new species of sandfly is described from limestone caves in Thailand. The inclusion of this species in the genus Chinius, which up until now was monospecific, is discussed. It is justified on the basis of characteristics of the head (eyes, pharynx, cibarium, complete interocular suture and length of the mouth pieces), thorax (rounded wings), abdomen (presence of trumpet glands on the tergites 4 and 5 of the male) and genitalia (morphology of the male genitalia and of the spermathecae in the female). Detailed descriptions and drawings are given. The wing of C. barbazani n. sp. lacks of vein R2 in both sexes. This anomaly, regarding to Phlebotominae, is discussed and considered as a probable autapomorphic regression. The differential diagnosis with Chinius junlianensis Leng, 1987, rests on a number of characteristics of the wing venation, antennal formula and the length of the male and female genital ducts, which are five times shorter in C. barbazani n. sp.