The red deer (Cervus elaphus) is one of the largest deer species. A male red deer is called a stag or hart, and a female is called a doe or hind. The red deer inhabits most of Europe, the Caucasus Mountains region, Anatolia, Iran, and parts of Western Asia. It also inhabits the Atlas Mountains of Northern Africa, being the only living species of deer to inhabit Africa. Red deer have been introduced to other areas, including Australia, New Zealand, the United States, Canada, Peru, Uruguay, Chile and Argentina. In many parts of the world, the meat (venison) from red deer is used as a food source.

The red deer is a ruminant, characterized by a four-chambered stomach. Genetic evidence indicates that the red deer, as traditionally defined, is a species group, rather than a single species, though exactly how many species the group includes remains disputed. The ancestor of the red deer probably originated in central Asia.

Although at one time red deer were rare in parts of Europe, they were never close to extinction. Reintroduction and conservation efforts, such as in the United Kingdom and Portugal, have resulted in an increase of red deer populations, while other areas, such as North Africa, have continued to show a population decline.

The red deer is the fourth-largest extant deer species, behind the moose, elk, and sambar deer. It is a ruminant, eating its food in two stages and having an even number of toes on each hoof, like camels, goats, and cattle. European red deer have a relatively long tail compared with their Asian and North American relatives. Subtle differences in appearance are noted between the various subspecies of red deer, primarily in size and antlers, with the smallest being the Corsican red deer found on the islands of Corsica and Sardinia and the largest being the Caspian red deer (or maral) of Asia Minor and the Caucasus Region to the west of the Caspian Sea.

The deer of central and western Europe vary greatly in size, with some of the largest deer found in the Carpathian Mountains in Central Europe. Western European red deer, historically, grew to large size given ample food supply (including people's crops), and descendants of introduced populations living in New Zealand and Argentina have grown quite large in both body and antler size. Large red deer stags, like the Caspian red deer or those of the Carpathian Mountains, may rival North American elk in size. Female red deer are much smaller than the males.

The male (stag) red deer is typically 175 to 250 cm (69 to 98 in) long from the nose to the base of the tail and typically weighs 160 to 240 kg (350 to 530 lb); the female (hind) is 160 to 210 cm (63 to 83 in) long and often weighs 120 to 170 kg (260 to 370 lb). The tail adds another 12 to 19 cm (4+1⁄2 to 7+1⁄2 in) and shoulder height is about 95 to 130 cm (37 to 51 in). In Scotland, stags average 201 cm (79 in) in head-and-body length and 122 cm (48 in) high at the shoulder and females average 180 cm (71 in) long and 114 cm (45 in) tall. Based on body mass, they are likely the fourth largest extant deer species on average, behind the moose, the elk and the sambar deer.

Size varies in different subspecies with the largest, the huge but small-antlered deer of the Carpathian Mountains (C. e. elaphus), weighing up to 500 kg (1,100 lb). At the other end of the scale, the Corsican red deer (C. e. corsicanus) weighs about 80 to 100 kg (180 to 220 lb), although red deer in poor habitats can weigh as little as 53 to 112 kg (120 to 250 lb).

The males of many subspecies also grow a short neck mane during the autumn. The male deer of the British Isles and Norway tend to have the thickest and most noticeable manes. Male Caspian red deer (C. e. maral) and Spanish red deer (C. e. hispanicus) do not carry neck manes. Male deer of all subspecies, however, tend to have stronger and thicker neck muscles than female deer, which may give them an appearance of having neck manes. Red deer hinds (females) do not have neck manes.