The Argentine fat-tailed mouse opossum (Thylamys sponsorius) is a species of opossum in the family Didelphidae. It is found in the eastern foothills of the Andes in northern Argentina and southern Bolivia. Its dorsal fur is gray brown to dark brown. Its ventral fur is gray-based except for the white to yellowish chest hairs. It is distinguished from Thylamys cinderella by its postorbital ridges. Thylamys cinderella has well-developed postorbital ridges in both juveniles and adults that extend laterally behind the eye sockets. Only adults of Thylamys sponsorius have fully developed postorbital ridges, and these do not extend laterally behind the eye sockets.