The buff-bellied fat-tailed mouse opossum (Thylamys venustus ) is a species of opossum in the family Didelphidae. It is found in the transitional and humid forests of northern Argentina and southern Bolivia. Its dorsal fur is cinnamon brown. Most of its ventral fur is gray-based, but its chest, throat, and the thoracic midline (the midline of the thorax, which ranges from the throat to the top of the abdomen) are not gray-based. The postorbital ridges are absent in the young and weakly developed in adults. There are four synonyms: Marmosa elegans venusta (Thomas, 1902), Marmosa elegans cinderella (Thomas, 1902, Tucumán Province), Marmosa elegans sponsoria (Thomas, 1921, Jujuy Province) and Marmosa janetta (Thomas, 1926, Tarija Department) with cinderella and sponsoria actually being one taxon and a subspecies of venustus ; janetta is the largest of all and has cream-white ventral pelage with plumbeous bases.

The threats are not exactly known; it occurs in an area currently being developed, and while its range overlaps several protected areas, it is uncertain if it occurs in any of them.