The mountain beaver is a North American rodent. It is the only living member of its genus, Aplodontia, and family, Aplodontiidae. It should not be confused with true North American and Eurasian beavers, to which it is not closely related; the mountain beaver is instead more closely related to squirrels, although its less-efficient renal system was thought to indicate greater relative antiquity for the species. There are seven subspecies of mountain beavers, six of which are found in California and three of which are endemic to the state.

The North American beaver is one of two extant beaver species, along with the Eurasian beaver. It is native to North America and has been introduced in South America (Patagonia) and Europe. The North American beaver is one of the official national wildlife of Canada symbols and is the official state mammal of Oregon and New York. North American beavers are widespread across the continental United States, Canada, southern Alaska, and some parts of northern Mexico.