Whereas previous treatments have considered Heteromys australis the only spiny pocket mouse present in Ecuador, morphological and morphometric analyses of specimens from Ecuador and southwestern Colombia reveal the presence of two species of the genus. Heteromys australis is distributed in evergreen forests from eastern Panamá and western Venezuela through Colombia to extreme northwestern Ecuador, where it inhabits wet, unseasonal areas of the Chocó and adjacent western slopes of the Andes. We here describe a new species, Heteromys teleus, found only in evergreen forests of central-western Ecuador, in areas less mesic and more seasonal than those characteristic of H. australis. Both species possess dark gray dorsal pelage, but H. teleus differs by larger (nonoverlapping) measurements of the hind foot and distinctive cranial proportions. Most notably, the rostrum of the new species is strikingly wide and massive, and the interparietal is narrow and rounded (in contrast to the wide, diamond-shaped interparietal of H. australis). The ranges of the two species together conform to the previously recognized Chocoan evergreen-forest fauna of western Colombia and northwestern Ecuador. However, the restriction of H. teleus to evergreen but seasonal forests of the southern Chocó (transitional between the relatively unseasonal evergreen forests of the central Chocó to the north and highly seasonal xeric regions to the south) is unique within currently recognized species of mammals. Biogeographic overviews hint at similar patterns in other groups, but more alpha-taxonomic research is necessary to evaluate mammalian distributional patterns in the region properly. Most suitable habitat for H. teleus has been converted to agricultural uses, and its current distribution is likely restricted to a handful of small-to-medium-sized forest patches.