The Isthmian mouse opossum (Marmosa isthmica) is a species of opossum in the family Didelphidae found in Colombia, Ecuador, and Panama. Foraging along branches and vines for fruit and insects, with the help of a prehensile tail, M. isthmica was formerly considered a subspecies of Robinson's mouse opossum (Marmosa robinsoni) and is supposed to be similar to it in habit, but following Rossi (2005) it is now deemed a species. In 1935 in the Panama Canal Zone, Enders observed Marmosa isthmica to build nests with leaves in a nestbox fixed to a tree.