Palaeocastor is an extinct genus of beavers that lived in the North American Badlands during the late Oligocene period to early Miocene. Palaeocastor was much smaller than modern beavers. There are several species including Palaeocastor fossor, Palaeocastor magnus, Palaeocastor wahlerti, and Palaeocastor peninsulatus. The animals first became known on grounds of their fossilized burrows, the "Devil's corkscrews".

Castoridae is a family of rodents that contains the two living species of beavers and their fossil relatives. A formerly diverse group, only a single genus is extant today, Castor. Two other genera of "giant beavers", Castoroides and Trogontherium, became extinct in the Late Pleistocene.