Marsupials are an infraclass of mammals living primarily in the Southern Hemisphere and carrying their young in a pouch. Well-known marsupials include kangaroos, koalas, possums, opossums, wombats and the Tasmanian devil. Marsupials represent the clade originating with the last common ancestor of extant metatherians. Like other mammals in the Metatheria, they are characterized by giving birth to relatively undeveloped young, often residing in a pouch with the parent for a certain time after birth. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central America, and one in North America, north of Mexico.

The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists. Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes. Most genetic and molecular evidence also supports grouping marsupials and placental mammals as a single clade, subclass Theria.