Marmosa constantiae O. Thomas, 1904, is a medium-sized didelphid commonly called the white-bellied woolly mouse opossum. It is a large, stocky mouse opossum with a relatively short snout and thick, woolly pelage, 1 of 6 members of the Marmosa subgenus Micoureus. The species is poorly known but relatively widespread in central South America, from Bolivia south to extreme northern Argentina and east to Paraguay and west-central Brazil. It is a nocturnal and arboreal inhabitant of humid and subhumid forests. Though known from relatively few specimens, the species has proved to be more widespread than previously thought and is not of special conservation concern. It is currently listed as “Least Concern” by the International Union for Conservation of Nature and Natural Resources.