The dwarf fat-tailed mouse opossum (Thylamys velutinus), also known as the velvety fat-tailed opossum, is an opossum species from South America. It is endemic to Brazil, where it is found in cerrado and caatinga habitats. Its head-and-body length ranges from , with an average of , and its tail length ranges from with an average . Its diet is 44% arthropods; 75% of its diet consists of animals and animal products. Its ventral fur is entirely gray-based. Its hind foot is less than , which is short for the genus.