Marsupialia, order of mammals belonging to the infraclass Metatheria, comprising some 280 living species, of which two-thirds are found in Australia. The Virginia or North American opossum (Didelphis virginiana) is the only marsupial found in Canada.

Marsupials differ from placental mammals in many anatomical details, especially in the reproductive system. In the vast majority of marsupials, no placenta is formed and the young are born in a premature state after a brief gestation. The young complete their development attached to a teat and protected by skin folds or a fully formed pouch (marsupium).

Marsupials and placental mammals evolved at about the same time in the Cretaceous period (144.2-65 million years ago) from a group of primitive mammals, the pantotheres. The oldest fossils are from the Upper Cretaceous (98.9-65 million years ago) of North America. Marsupials are found in North and South America and Australia, which they may have colonized, via Antarctica, before continental drift (see Plate Techtonics) moved Australia to its present location.