The white-bellied slender opossum (Marmosops noctivagus) is a species of opossum from South America. It is found in Bolivia, Brazil, Ecuador and Peru.
Though previously regarded as a species, M. dorothea, Dorothy's slender opossum, is a synonym of M. noctivagus. On the other hand, M. ocellatus, a junior synonym of M. dorothea, is now recognized as a separate true species.