The hemipenial morphology of nine species in Ninia is described for the first time. A total of 65 hemipenes of all recognized species of Ninia were examined. As a result of the evaluation of the hemipenial and caudal morphology, Ninia diademata labiosa is elevated to full species status and a new species is described from 240 m above sea level in the Northern Range of Trinidad. The new species exhibits several diagnostic features, including (1) a primary temporal scale that is extremely long and contacts the upper labial scales 5–7; (2) a temporal scale formula 1 2, with the primary temporal scale three times longer than the lower posterior temporal; and (3) forked hemipenes. The male genitalia of Ninia are excellent indicators of species limits and an effective tool to clarify the identity of cryptic species in the genus.