The Key Deer (Odocoileus virginianus clavium) is an endangered deer that lives only in the Florida Keys. It is a subspecies of the White-tailed deer (O. virginianus).

This deer can be recognized by its characteristic size, smaller than all other White-tailed deer. Adult males (known as bucks) usually weigh 25–34 kilograms (55–75 lb) and stand about 76 centimetres (30 in) tall at the shoulder. Adult females (does) usually weigh between 20–29 kilograms (44–64 lb) and have an average height of 66 centimetres (26 in) at the shoulders. The deer is a reddish-brown to grey-brown in color. Antlers are grown by males and shed between February and March and regrown by June. When the antlers are growing, they have a white velvet coating. The species otherwise generally resembles other white-tailed deer in appearance.

Key Deer easily swim between islands.

Living close to humans, the Key Deer has little of the natural fear of man shown by most of their larger mainland cousins. The deer are often found in residents' yards and along roadsides where tasty plants and flowers grow. This often results in car-to-deer collisions, as the deer are more active (and harder to avoid) at night. It is not unusual to see them at dusk and dawn, especially on lightly-inhabited No Name Key, and in the less-populated northern areas of Big Pine Key.

Mating may occur at any time of the year but usually peaks during mating season (rut) from September to November. Key Deer have a relatively low reproductive rate, averaging 1.08 young (fawns) per adult doe per year.

The range of the Key Deer originally encompassed all of the lower Florida Keys (where standing water pools exist), but is now limited to a stretch of the Florida Keys from about Sugarloaf Key to Bahia Honda Key.

The islands of Big Pine, Cudjoe, West Summerland, Big Torch, Howe, Little Pine, Little Torch, Middle Torch, No Name, and Sugarloaf are the only places where the deer permanently reside. The deer also can be found on the islands of Annette, Big Munson, Little Munson, Johnson, Knockemdown, Mayo, Porpoise, Ramrod, Toptree Hammock, Wahoo, Water Key (east) and Water Keys (west), but they are only present there in the dry season when there is a shortage of a fresh water.

Key Deer use all islands during the wet season when drinking water is more generally available, retreating to islands with a perennial supply of fresh water in dry months.

Key Deer inhabit nearly all habitats within their range, including pine rocklands, hardwood hammocks, mangroves, and freshwater wetlands. The species feed on over 150 types of plants, but mangroves (red, white, and black) and thatch palm berries make up the most important part of their diet. Pine rockland habitat is important as well because it is often the only reliable source of fresh drinking water (Key Deer can tolerate drinking only mildly brackish water). Habitat destruction due to human encroachment cause many deer to feed on non-native ornamental plants.

It is believed that the Key Deer is a subspecies of White-tailed deer which migrated to the Florida Keys from the mainland over a land bridge during the Wisconsin glaciation. The earliest known written reference to Key Deer comes from the writings of Hernando de Escalante Fontaneda, a Spanish sailor shipwrecked in the Florida Keys and captured by Native Americans in the 1550s.

Deer were hunted as a food supply by native tribes, passing sailors, and early settlers. The hunting of Key Deer was banned in 1939, but widespread poaching and habitat destruction caused the subspecies to plummet to near-extinction by the 1950s. The National Key Deer Refuge, a federally administered National Wildlife Refuge operated by the Wildlife Service, was established in 1957.

Recent population estimates put the population between 300 and 800, putting it on the list of endangered species. Road kills from drivers on U.S. 1, which traverses the deer's small range, are also a major threat, averaging between 30 and 40 kills per year, 70% of the annual mortality.

However, the population has made an encouraging rise since 1955, when population estimates ranged as low as 25, and appears to have stabilized in recent years. Still, recent human encroachment into the fragile habitat and the deer's relatively low rate of reproduction point to an uncertain future for the species.

Conservation efforts include the establishment of the National Key Deer Refuge, which consists of approximately 8,500 acres (34 km2) on Big Pine, No Name Key and several smaller uninhabited islands. Not all of the refuge lands are protected as public lands; despite extensive efforts of the Refuge to purchase these private habitat lands for protection in the refuge, about 5,000 acres (20 km2) currently remain in private ownership and can potentially be developed. About 1,000 acres (4.0 km2) of this privately developable land is on Big Pine Key and No Name Key, which is the central population area for the deer. In 2006, a Habitat Conservation Plan was enacted by Monroe County and the U.S. Fish and Wildlife Service which will limit development in primary habitat and provide for additional habitat purchases over the next 15 years. At the end of this period, however, most of the 1,000 acres (4.0 km2) of privately owned habitat land on Big Pine and No Name Keys will still be open for further development. Thus while the short term promises some cushion from extinction, the long-term prospects for the deer remain in doubt. The refuge also provides rules for the control of feral dogs, which often attack the deer, and for the prohibition of feeding of the deer, which lowers their resistance to natural cycles; however, historically, due to local political pressures, these rules have been poorly enforced. A portion of U.S. Route 1 was also elevated in 2003 to allow the deer to pass safely beneath the roadway, in an attempt to lessen the chance of road kills. However, no decrease in total traffic deaths has been seen. Biologists have recently begun relocating some Key Deer from Big Pine Key to other islands, since Big Pine Key's population has reached its sustainable limit.