The beaver is usually about 76 cm (about 30 in) long and stands less than 30 cm (12 in) high. The broad, flat, scaly tail is about 25 cm (about 10 in) long and serves as a warning signal when slapped against the water, as a support when the beaver is standing on its hind legs, and as a rudder while swimming. The body is plump, the back arched, the neck thick, the hind feet webbed, and all the digits clawed.

The fur is usually reddish-brown above and lighter or grayish below. The eyes are small, and the nostrils closable. The skull is massive, with marked ridges for fixing the muscles that work the jaws. The two front teeth on either jaw are like those of other rodents, wearing away more rapidly behind so as to leave a sharp, enamelled chisel edge. With these, the beaver can cut down large trees, usually selecting those 5 to 20 cm (2 to 8 in) in diameter, though it can fell trees as large as 76 cm (30 in).

Beavers have a pair of anal scent glands, called castors, that secrete castoreum, likely for marking territories. The animals are monogamous, may live 20 years or more, and females usually have one litter a year of two to four young.