We expand upon recent studies on relationships within the Oryzomyini, in particular, those involving taxa currently assigned to the genus Sigmodontomys. In recent years, Sigmodontomys has been treated as including 2 species, alfari (J. A. Allen, 1897) and aphrastus (Harris, 1932), but throughout their complicated taxonomic history both species also have been placed in the genus Oryzomys, and alfari independently in Nectomys. Using morphological (98 external, cranial, dental, and postcranial) and molecular (nuclear interphotoreceptor retinoid–binding protein gene and mitochondrial cytochrome-b and ribosomal 12S RNA genes) characters, we infer the phylogenetic position of these 2 species within Oryzomyini. We document that alfari and aphrastus do not form a monophyletic group. Sigmodontomys alfari is most closely related to Melanomys, and aphrastus is either the sister to that clade, or to the extinct Caribbean genus Megalomys. Thus, aphrastus is best regarded as representing a new genus, which is described and named herein. This new genus falls within the Sigmodontomys–Melanomys–Aegialomys–Nesoryzomys clade, which forms a monophyletic group of mainly southern Central American and northern South American taxa primarily restricted to lowland to midelevation montane trans-Andean habitats and possessing a marked ability to cross expanses of salt water. The new genus occurs at middle elevations from north-central Costa Rica to northwestern Ecuador and along with some populations of Aegialomys and Melanomys occupies the highest elevations for members of this group.