The head, trunk, and tail of the platypus are broad and flattened and covered with thick dark brown fur. The muzzle is shaped like a duck's bill and is soft and rubbery. It contains ridges used for crushing food; the animal has no teeth. The eyes are small and there are no external ears. The five-toed feet are webbed. The heel of the adult male bears a hollow spur connected to a poison-secreting gland; this spur is probably used as a weapon. Females lose their spurs at about one year of age. The adult male platypus is about 2 ft (60 cm) long, including the 5 or 6 in. (13–15 cm) tail; it weighs about 4 lb. The female is slightly smaller.

The platypus is found from tropical swamps at sea level to cold lakes at altitudes of 6,000 ft (1,830 m). Its diet consists entirely of small freshwater animals dredged from muddy bottoms. Prey captured underwater are stored in cheek pouches and eaten at the surface or on land.

Platypuses live in pairs in simple burrows in stream banks, except during the breeding season, when the female makes a separate and more elaborate burrow containing a nesting chamber approached by a long tunnel. One, two, or three eggs are laid at a time and are incubated, in birdlike fashion, by the female. The female lacks nipples, and the young lick milk from the fur around the many small abdominal openings of the mammary glands. The platypus is classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Monotremata, family Ornithorhynchidae.