A new species of short-tailed opossum (Monodelphis) is described from the lowland forest of southeastern Peru. The new species is similar and more related to Monodelphis adusto Thomas, 1897, but is comparatively larger, and lighter in color. Other species known to occur in the area (M. emiliae and M. glirina) have no closer resemblance to the new species. Other characters useful to distinguish this new species are a heavy, broad, and extremely flattened skull, with a conspicuous sagittal crest. The premaxilla is short and the upper incisor rows are wide apart. During the comparisons of the new species with M. adusta, it became clear that two subspecies of the latter could be recognized and morphologically diagnosed: Monodelphis a. adusta and M. a. peruviana. Their distributions are limited to the south and to the north, respectively, by the Amazon River. The name M. melanops, described from Panama is a junior synonym of M. a. adusta, but M. osgoodi, described from the highlands of Bolivia is a valid species, and probably has no close relationships to either M. adusta or the new species here described.