The Arizona Gray Squirrel (Sciurus arizonensis) is a species of rodent in the Sciuridae family. It is found in Mexico and the United States. Its natural habitat is temperate forests. It is threatened by habitat loss.

Walnuts are a favorite food of Arizona Gray Squirrels, and when they find an abundance, the squirrels soon stain their faces, paws, and undersides a distinct brownish-orange from walnut juice. Other foods eaten may include fungi, acorns, juniper berries, pine seeds, and tree flowers and buds. This species is quiet and secretive and rarely seen. It is not very common and has a limited geographic distribution. Within its range, Arizona Gray Squirrels prefer broadleaf forests along rivers, which commonly occur in canyon bottoms.

It's close relative, the Western Gray Squirrel (Sciurus griseus) is a tree squirrel found along the western coast of the United States and Canada.

At various times and places, this species has also been known as the Silver-gray Squirrel, the California Gray Squirrel, the Oregon Gray Squirrel, the Columbian Gray Squirrel, the Banner-tail, and also simply as the Gray Squirrel. There are three geographical subspecies: Sciurus griseus griseus (central Washington to the western Sierra Nevada in central California); S. g. nigripes (from south of San Francisco Bay to San Luis Obispo County, California; and S. g. anthonyi, which ranges from San Luis Obispo to south-central California).

The Western Gray Squirrel was first described by George Ord in 1818 based on notes taken by Lewis and Clark at The Dalles in Wasco County, Oregon.

Compared with the Eastern Gray Squirrel S. carolinensis or the Eastern Fox Squirrel (which have been introduced into its range), these squirrels are shy, and will generally run up a tree and give a hoarse barking call when disturbed. Weights vary from about 400 g to nearly 1 kg, and length (including tail) from 45 to 60 cm. It is the largest native tree squirrel in the western coastal United States. Western Gray Squirrels exhibit a form of coloration known as counter shading. The dorsal fur is a silver gunmetal gray, with pure white on the underside; there may be black flecks in the tail. Ears are large but without tufts. The ears turn reddish-brown at the back in the winter. The tail is long and typically very bushy. Tree squirrels undergo a complete head-to-tail molt in the spring and a rump-to-head molt in the fall. Tail hair is replaced only in the spring. Also, nesting mothers will use their tail hair to line birthing nests.

Western Gray Squirrels mate over an extended period ranging from December through June. Young are born after approximate 44 day gestation period. Juveniles emerge from nests between March and mid-August. Litter sizes range from 1 to 5. Kits remain in the nest for a longer period that other squirrels, and are relatively slow in development, another detriment to the species when it is in conflict with other, more-rapidly independent squirrels. Young gray squirrels will not leave the nest until 6 months or more. They have "furled" tails which will not reach fullness until adulthood. This is a good indicator of age and maturity. Mother squirrels often have a harassed, overworked look, complete with bruised and battered nipples. Mating squirrels can be very sadistic and will bite and injure each other. Females can be quite territorial, and will chase others away and have fairly violent altercations between themselves.

Western Gray Squirrels are forest dwellers, and can be found at elevations up to at least 2000 m. Time on the ground is spent foraging, but they prefer to travel distances from tree to tree. They are strictly diurnal, and feed mainly on seeds and nuts, particularly pine seeds and acorns, though they will also take berries, fungus and other soft food. Pine nuts and acorns are considered critical foods because they are very high in oil and moderately high in carbohydrates, which help increase the development of body fat. They feed mostly in trees and on the ground. They generally forage in the morning and late afternoon for acorns, pine nuts, new tree buds, and fruits. When on alert, they will spread their tails lavishly, creating an umbrella effect that shields them and possibly provides cover from overhead predators. They are scatter-hoarders making numerous caches of food when it is abundant, and thus contribute to the seed dispersion of their food trees. Although they show relatively good scent relocation abilities, some food caches will never be reclaimed, becoming seedlings in the spring. They do not hibernate, but become less active during the winter. Like many prey animals, they depend on auditory alerts from other squirrels or birds to determine safety. Once an alarm call is transmitted, those present will join in, and the trees become a cacophony of barking squirrels. Tree squirrels are prey for bobcats, hawks, eagles, and mountain lions.

Nests can be seen in tall trees, built from sticks and leaves wrapped with long strands of grass. There are two stick nest types made by the Western Gray Squirrel, although one is not properly termed a "nest." The first is a large, round, covered shelter nest for winter use, birthing, and rearing young. The second is more properly termed a "sleeping platform," a base for seasonal or temporary use. Both types are built with sticks and twigs and are lined with leaves, moss, lichens and shredded bark. The birthing nest may be lined with tail hair. The nest may measure 43 - 91 cm (17 to 36 inches) in length and up to 46 cm (18 inches) in height. It is usually built within the top third of the tree. Young or traveling squirrels will also "sleep rough" when weather permits, balanced spread-eagled on a tree limb high above the forest floor. This attitude is also adopted for cooling purposes in hot weather (a behaviour also observed in Raccoons).

The Western Gray Squirrel was listed as a threatened species in Washington state in 1993. Populations of the Western Gray Squirrel have not recovered from past reductions. They are being threatened with habitat loss, road-kill mortality and disease. Habitat has been lost due to urbanization and catastrophic wild fires. Previous areas of forest have been degraded by fire suppression and overgrazing, allowing the invasion of Scot's Broom. Notoedric mange, a disease caused by mites, becomes epidemic in Western Gray Squirrel populations and is a major source of mortality. Other species of Eastern Gray Squirrels, fox squirrels, California ground squirrels and Wild Turkeys are expanding and compete with the Western Gray.

Listed as extirpated in some California areas, the Western Gray Squirrel in southern California is generally found only in the mountains and surrounding foothill communities. Local rehabilitation experts recount the Eastern Fox Squirrels were released in urban regions Los Angeles in the 1970s, and these aggressive cousins drove the more reclusive Western Grays back into the mountains, where competition was not so strong. This non-native species introduction appears to be the largest threat in the southern California area.