Pirnodus (Oribatida, Oripodidae) currently comprises seven species, collectively distributed in the tropical and southern Palaearctic areas. A new arboreal species – P. arboreus sp. nov. – is described, based on adults collected from branches of different trees in the Cat Tien National Park, southern Vietnam. The new species is characterized by distinct sexual dimorphism, e.g., interlamellar seta long, setiform in females versus medium-sized, phylliform in males, posterior part of notogaster not depressed in females versus depressed in males, notogastral seta h1 well developed, setiform in females versus vestigial in males, notogastral seta lm located medial to la in females versus anteromedial in males, genital and anal apertures distanced from each other in females versus genital aperture shifted to anal aperture and connected to it in males, and dorsal side of tarsi I and II without tooth in females versus with tooth in males. Summary of distribution and habitats of the known species of Pirnodus are provided.