Castoroides (from Latin castor (beaver) and -oides (like)), or the giant beaver, is an extinct genus of enormous, bear-sized beavers that lived in North America during the Pleistocene. Two species are currently recognized, C. dilophidus in the Southeastern United States and C. ohioensis in most of North America. C. leiseyorum was previously described from the Irvingtonian age but is now regarded as an invalid name. All specimens previously described as C. leiseyorum are considered to belong to C. dilophidus.

Castoroides species were much larger than modern beavers. Their average length was approximately 1.9 m (6.2 ft), and they could grow as large as 2.2 m (7.2 ft). The weight of the giant beaver could vary from 90 kg (198 lb) to 125 kg (276 lb). This makes it the largest known rodent in North America during the Pleistocene and the largest known beaver. Recent analyses suggest that they weighed less, closer to 77 kg (170 lb), but this is disputable.

The hind feet of the giant beaver were much larger than in modern beavers, while the hind legs were shorter. The tail was longer and may not have been paddle-shaped as in modern beavers. It can only be assumed that its feet were webbed as in modern species. The skull structure of the giant beaver suggests that it participated in extended underwater activity, thanks to the ability to take more oxygen into its lungs.

One of the defining characteristics of the giant beaver was their incisor teeth, which differed in size and shape from those of modern beavers. Modern beavers have incisors with smooth enamel, while the teeth of the giant beaver had a striated, textured enamel surface. Their teeth were also much larger, up to 15 cm (6 in) long.

There are two known species:

These two species of giant beaver (genus Castoroides) are not close relatives to modern beavers (genus Castor), with modern beavers and Castoroides suggested to have split around 16-20 million years ago based on fossils and genomic estimates. Castoroides typifies the extinct subfamily Castoroidinae, which forms a North American lineage beginning with the Hemingfordian genus Monosaulax, followed by Eucastor, Dipoides, and Procastoroides, to finally culminate and go extinct with Castoroides.

Castoroides fossils were first discovered in 1837 in a peat bog in Ohio, hence the species epithet ohioensis. Castoroides had cutting teeth up to 15 cm-long with prominently-ridged outer surfaces. These strong enamel ridges would have acted as girders to support such long teeth. Further, the deep masseteric fossa of the lower jaw suggests a very powerful bite. Remains of the giant beaver, along with Paleo-Indian artifacts and the remains of the flat-headed peccary, giant short-faced bear, and the stag moose were found in the Sheriden Cave in Wyandot County, Ohio.

Fossils of Castoroides are concentrated around the midwestern United States in states near the Great Lakes, particularly Illinois and Indiana, but specimens are recorded from Alaska and Canada to Florida. In Canada, fossils of this species are commonly found in the Old Crow Basin, Yukon, and single specimens are known from Toronto, Ontario and Indian Island, New Brunswick. A hitherto overlooked 1891 record of a Castoroides skull from near Highgate, Ontario is the earliest for Canada. In Old Crow region, Castoroides fossils occur in deposits of the Sangamonian interglacial.