﻿AbstractThe snorkel snail genusRhiostomaBenson, 1860 is comprised of terrestrial cyclophorid snails with wide-ranging species diversity and radiation in Southeast Asia. The typical characters of the genus are a depressed shell, a detached and descending portion of the last whorl with a distinctive peristomal breathing device attached, and a calcareous cup-shaped operculum. Herein, we have revised the systematics of extant species based on shell morphology combined withCOIbarcoding. From these thirty recognised species, twelve are described as new to science:R.?amarapuraensesp. nov.,R.anceyisp. nov.,R.breviocollarsp. nov.,R.ebenozosterasp. nov.,R.cheliopegmasp. nov.,R.furfurosumsp. nov.,R.gnomus,sp. nov.,R.lannaensesp. nov.,R.laoensesp. nov.,R.platymorphasp. nov.,R.rhothonotaphrosasp. nov., andR.tigrinasp. nov.All conchological characters are provided via illustrations of type specimens and living snails, and descriptions of the shells and radulae. Phylogenetic analysis based on partialCOIgene sequences strongly supports the designated morphospecies and a monophyleticRhiostoma, confirming that all pterocyclinid snails with a calcareous, cup-shaped operculum belong to the same clade. A high intra-specific divergence was observed inR.jalorensisandR.houseipopulations from locations in close proximity, suggesting a lower dispersal and higher level of isolation. The low inter-specific divergence found inR.hainesi,R.samuiense,R.asiphon, andR.rhothonotaphrosasp. nov.supports their recent diversification and local adaptation, and is congruent with their marked morphological differences. Finally, nine formerlyRhiostoma-placed species were reclassified into either the genusCyclotusor the genusOpisthoporus.