Mouse opossums of the genus Marmosa Gray (Didelphidae) represent a complex of taxa with poorly understood affinities. Species now placed within this genus are mostly the leftovers from previous taxonomic arrangements. Even the most recent and complete revisions have not fully resolved the relationships among Marmosa and the many taxa previously included in Marmosa or listed as synonyms of it. Herein, we review and discuss one of these taxa, Stegomarmosa, based on new specimens that significantly increase our knowledge of the morphological diversification of the group. Until recently, this genus/subgenus has been known only from a single specimen taken more than 50 years ago in southeastern Peru. In 1997-1998, six additional specimens were collected at two little-separated localities almost 200 km NW of the type locality. We include a detailed diagnosis of the genus-group Stegomarmosa and its type species, M. andersoni, and update the distribution for and provide new information on the natural history of the latter. We also discuss published claims, concerning geographic range, that may be inaccurate and misleading. Finally, we scored the morphological characters used in the most recent phylogenetic analyses of the didelphids, in order to explore the affinities of M. andersoni and to determine the distinctiveness of the taxon Stegomarmosa.