Black squirrels are a melanistic subgroup of squirrels with black coloration on their fur. The phenomenon occurs with several species of squirrels, although it is most frequent with the eastern gray squirrel (Sciurus carolinensis) and the fox squirrel (Sciurus niger). Black morphs of the eastern gray and fox squirrels are the result of a variant pigment gene. Several theories have surfaced as to why the black morph occurs, with some suggesting that the black morph is a selective advantage for squirrels inhabiting the northern ranges of the species, with the black fur providing a thermal advantage over its non-melanistic counterpart.

Black squirrels share the same natural range as their non-melanistic counterparts. Black morphs of eastern gray squirrels occur most frequently in the northern portion of its range around the Great Lakes Basin. Conversely, black morphs of fox squirrels typically occur most frequently in the southeastern portions of the species' natural range, the southeastern United States. Although they are found more frequently in those regions, the coloration remains uncommon in most areas that these species inhabit. However, black morphs of eastern gray squirrels form the majority of the species' population in the Canadian province of Ontario, and the U.S. state of Michigan. In addition to their natural range, black morphs of eastern gray squirrels were also introduced into other areas of Canada, the United Kingdom, and the United States during the 19th and 20th centuries.

Several municipalities and post-secondary schools in the United States have adopted a black squirrel for branding purposes, using it as a symbol and/or mascot. Some municipalities that have adopted the black squirrels as a symbol for their community have also passed ordinances that discourage attempts to threaten them.

The black coloration in both eastern gray squirrels and fox squirrels is believed to stem from a variant pigment gene. A study published by FEBS Letters in 2014 demonstrated how a pigment gene missing a piece of DNA, can be a determinant of an eastern gray squirrel's coat. The emergence of black fur in the eastern gray squirrel is believed to be the result of the 24 bp deletion from their melanocortin 1 receptor (MC1R) gene; with the specific allele referred to as MC1R∆24. A study published by BMC Evolutionary Biology pointed to evidence that the variant pigment gene originated from the black fox squirrel, and was later passed on to eastern gray squirrels as a result of interspecies mating; given that the variant gene in both species were identical. Black coat color is caused by a 24 base pair deletion in MC1R in the western population of fox squirrels and by a point mutation in the agouti-signaling protein gene in the southeastern population.

Black morphs may also occur with Columbian ground squirrels, Eurasian red squirrels, Richardson's ground squirrels, and western gray squirrels, although it is far more unusual for the latter to display color polymorphism. No association between melanism and variations in their MC1R was found in Eurasian red squirrels; with researchers suggesting that the different color variations (including black morphs) in Eurasian red squirrels, and fox squirrels being a polygenic result. Melanism with Richardson's ground squirrels is due to recessive genes.

With regard to black squirrels and melanism, two major theories dominate the literature, that its frequency is the result of crypsis, and/or the result of thermoregulation.

It has been theorized that non-melanistic gray squirrels have a concealment advantage in forests dominated by deciduous trees, while black squirrels hold a concealment advantage in forested areas in the northern portions of its range, where conifer trees are more prevalent. The theory is based on the idea that forests where coniferous trees are predominant block more sunlight from reaching the forest below, providing a dimly-lit habitat in which a darker-coated squirrel could better conceal itself compared to its lighter counterpart. It is also suggested that non-melanistic squirrels have a concealment advantage over their melanistic counterparts in deciduous forests because deciduous trees shed their leaves on a seasonal basis, illuminating the forested area below it during the winter season. A study conducted in 1989 on melanistic fox squirrels found the non-melanistic coloration better for concealment while the squirrel was still, but a melanistic coloration provided better concealment for when it was in motion.

The frequency of black morph eastern gray squirrels is thought to have been once relatively common throughout the eastern gray squirrel range, although their frequency and population have dwindled since the 1700s. It has been suggested that their population declined due to extensive deforestation and the hunting of squirrels for their meat and pelts; with the newly changed environment providing non-melanistic gray-colored squirrels an advantage in concealment. However, the theory that the black morphs squirrels were more prevalent prior to the 1700s, and that deforestation led to their decline has been challenged by some researchers. One study found a high frequency of black eastern gray squirrels lived in rural southern Ontario, an area primarily made up of farmland.