The Water Opossum or Yapok (Chironectes minimus) is a marsupial of the family Didelphidae. It is found in freshwater streams and lakes in Mexico, Central and South America to Argentina. It is 27-32.5 cm long, with a 36-40 cm long tail. It is the only marsupial to have adapted to an aquatic life. Its long tail aids its swimming, while the broadly webbed hind feet are used for propulsion. It lives in bankside burrows, emerging after dusk to swim and search for fish, crustaceans and other aquatic animals, which it eats on the bank.

The opossum mates in December and a litter of about 5 young is born 2 weeks later. A strong ring of muscle makes the pouch watertight, so the young remain dry, even when the mother is totally immersed in water. The male also has a pouch, where he places his genitalia before swimming.