Monodelphis ronaldi is a short-tailed opossum currently known only by the holotype captured in Madre de Dios, Peru. The relationships of this species with other congeners are unknown, although it has been hypothesized to belong to the subgenus Mygalodelphys. Here, we report six additional specimens of M. ronaldi from the Alto Purús, Ucayali department, Peru, extend the range of the species about 240 km eastward, examine ontogenetic variation, and analyze its phylogenetic relationships using sequences of the mitochondrial Cytochrome-b gene. Our analyses recovered M. ronaldi as the sister species of M. handleyi, confirming its position within the subgenus Mygalodelphys. The genetic distance between them was almost 6%, besides several morphological traits differentiating both species. However, some morphological traits previously proposed as diagnostic were found to vary with age, so an emended diagnosis for M. ronaldi is provided. Additionally, we include new information about the habitat, geographic range, and natural history of this elusive species.