The giant beaver was the largest rodent in North America during the last ice age. It was about 2.5 m (8 ft.) long and is estimated to have weighed 60 to 100 kg (132 to 220 lb.)—the size of a black bear.

As its common name implies, the giant beaver looked generally similar to the modern beaver, but was considerably larger. By comparison, however, its hind feet were greatly enlarged. It is uncertain as to whether the tail was similar to that of the modern beaver (broad and flattened, like a paddle), because the tails are made of soft tissue, which decays and disappears. For this same reason, we can only assume that its feet were also webbed.

Other differences are seen in their teeth. The cutting teeth (incisors) of the giant beaver were up to 15 cm (6 in.) long. They had prominently ridged outer surfaces and blunt, rounded tips, unlike the chisel-like tips of the modern beaver's incisors. The giant beaver's cheek teeth looked much like those of today's capybara. They had S-shaped enamel patterns on their grinding surfaces. The teeth were well adapted to grinding up the plants that made up the animal's diet.

Despite their general similarities, the giant beaver and the modern beaver are not close relatives. The giant beaver is classified in the genus Castoroides, while the modern beaver belongs to Castor. There were two species of giant beaver: Castoroides ohioensis and Castoroides leiseyorum.

The oldest giant beaver fossils (of Castoroides leiseyorum from Florida, U.S.A.) are 1.4 million years old. In Canada, fossils of Castoroides ohioensis—probably from the last interglacial age (about 130 000 years ago)—were found in Toronto, Ontario, and in the Old Crow Basin of Yukon. Giant beavers were most abundant just south of the Great Lakes, in what is now Indiana and Illinois, U.S.A.

The earliest Canadian record is of a skull that was found with the Highgate Mastodon in southern Ontario in 1891. The giant beaver died out in North America about 10 000 years ago, along with mammoths, mastodons and ice-age horses. Competition from the modern beaver, as well as reduction of its habitat, may have led to its demise.