The Fox Squirrel (Sciurus niger) is the largest species of tree squirrels native to North America. They are also sometimes referred to as the Stump-eared Squirrel, Raccoon Squirrel, or Monkey-faced Squirrel. They are sometimes mistaken for Eastern Gray Squirrels by casual observers in those areas where both species co-exist, despite the differences in size and coloration.

The Fox Squirrel's natural range extends throughout the eastern United States, excluding New England, north into the southern prairie provinces of Canada, and west to the Dakotas, Colorado, and Texas. They have been introduced into Northern California. While very versatile in their habitat choices, fox squirrels are most often found in forest patches of 400,000 square metres or less with an open understory, or in urban neighborhoods with trees. They thrive best among trees such as oak, hickory, walnut and pine that produce winter-storable foods like nuts. Western range extensions in Great Plains regions such as Kansas are associated with riverine corridors of cottonwood. A subspecies native to several eastern U.S. states, the Delmarva fox squirrel, Sciurus niger cinereus, is a listed endangered species.

Total body length measures 45 to 70 cm (17.71" - 27.55"), tail length is 20 to 33 cm(7.87" - 13") , and they range in weight from 500 to 1000 grams (1.1 lbs - 2.2 lbs.). There is no sexual dimorphism in size or appearance. Individuals tend to be smaller in the west. There are three distinct geographical phases in coloration: in most areas the animals are brown-grey to brown-yellow, while in eastern regions such as the Appalachians there are more strikingly-patterned dark brown and black squirrels with white bands on the face and tail. In the south can be found isolated communities with uniform black coats.

Fox Squirrels depend primarily on tree seeds for food, but they are generalist eaters and will also consume buds and fruits, cultivated grain, insects, birds' eggs, and small lizards. Cannibalism has been reported, but should be considered very rare. In their regular diet of nuts, fox squirrels are classic scatter-hoarders that bury caches of nuts in dispersed locations, some of which are inevitably left unretrieved to germinate.

Fox Squirrels are strictly diurnal, non-territorial, and spend more of their time on the ground than most other tree squirrels. They are still, however, agile climbers. They construct two types of homes called "dreys", depending on the season. Summer dreys are often little more than platforms of sticks high in the branches of trees, while winter dens are usually hollowed out of tree trunks by a succession of occupants over as many as 30 years. Cohabitation of these dens is not uncommon, particularly among breeding pairs.

There are two breeding seasons, one peaking in December and the other in June. The young are blind, without fur and helpless at birth. They become independent at about three months and maturity is reached after one year. Their maximum life expectancy is 12.6 years for females and 8.6 years for males. Humans, hawks, snakes and bobcats prey on the squirrels.

They are gregarious and apparently playful, often chasing each other up and down trees and across yards and clearings. They have a large vocabulary, consisting most notably an assortment of clucking and chucking sounds, not unlike some "game" birds, and they warn the listening world of approaching threats. In the spring and fall, groups of fox squirrels clucking and chucking together can make a small ruckus. They are impressive jumpers, easily spanning fifteen feet in horizontal leaps and free-falling twenty feet or more to a soft landing on a limb or trunk.

Fox Squirrels are also known for being living fossils, skeletally very similar to remains of the oldest-known squirrel, Protosciurus, from the late Oligocene and early Miocene epochs.