A new species of the didelphid marsupial genus Marmosops is described from the Pakaraima Highlands of western Guyana and from two highland sites in eastern Venezuela. All known specimens were collected on sandstone table mountains (eroded fragments of the Roraima Formation) in the eastern subregion of Pantepui. The new species, M. pakaraimae, is one of only seven mammals known to be endemic to Pantepui, and phylogenetic analyses of cytochrome- b sequence data indicate that its sister taxon is M. parvidens, a geographically adjacent lowland species. Our results, together with those from phylogenetic studies of other Pantepui endemic mammals, suggest that at least some of these highland taxa evolved from lowland species in the late Cenozoic and are neither ancient relicts of tepui vicariance nor the descendents of long-distance-dispersing Andean progenitors.