New phylogenomic analyses of South American arboreal echimyids show that there are three species within the genus Toromys (T. grandis, T. rhipidurus, and T. sp. nov.) and that the genera Pattonomys and Toromys form a clade that is the sister group to the other three genera of arboreal Echimyini (Echimys, Makalata, and Phyllomys). The arboreal echimyid species Pattonomys occasius is deeply divergent from other species of Pattonomys and from members of other extant named genera, although it shares a sister relationship with Toromys. We erect a new genus, Leiuromys, for it. To clarify the relationships among the species within these three genera, we describe them with molecular and morphological characters. Because most members of these genera have never been reviewed or diagnosed at the species level, we do so now, and we describe a new species, Toromys albiventris, from the upper Ucayali basin of Peru. We illustrate taxa and morphological features not before figured in publications.