The black wildebeest or white-tailed gnu (Connochaetes gnou) is one of the two closely related wildebeest species. It is a member of the genus Connochaetes and family Bovidae. It was first described in 1780 by Eberhard August Wilhelm von Zimmermann. The black wildebeest is typically 1.7–2.2 m (5 ft 7 in – 7 ft 3 in) in head-and-body length, and the typical weight is 110–180 kg (240–400 lb). Males stand about 1.11–1.21 m (3 ft 8 in – 4 ft 0 in) at the shoulder, while the height of the females is 1.06–1.16 m (3 ft 6 in – 3 ft 10 in). The black wildebeest is characterised by its white, long, horse-like tail. It also has a dark brown to black coat and long, dark-coloured hair between its forelegs and under its belly.

The black wildebeest is an herbivore, and almost the whole diet consists of grasses. Water is an essential requirement. The three distinct social groups are the female herds, the bachelor herds, and the territorial bulls. They are fast runners and communicate using a variety of visual and vocal communications. The primary breeding season for the black wildebeest is from February to April. A single calf is usually born after a gestational period of about 8.5 months. The calf remains with its mother until her next calf is born a year later. The black wildebeest inhabits open plains, grasslands, and karoo shrublands.

The natural populations of black wildebeest, endemic in the southern part of Africa, were almost completely exterminated in the 19th century, due to their reputation as pests and the value of their hides and meat, but the species has been reintroduced widely from captive specimens, both in private areas and nature reserves throughout most of Lesotho, Eswatini, and South Africa. The species has also been introduced outside its natural range in Namibia and Kenya.

The scientific name of the black wildebeest is Connochaetes gnou. The animal is placed in the genus Connochaetes and family Bovidae and was first described by German zoologist Eberhard August Wilhelm von Zimmermann in 1780. He based his description on an article written by natural philosopher Jean-Nicolas-Sébastien Allamand in 1776. The generic name Connochaetes derives from the Greek words κόννος, kónnos, "beard", and χαίτη, khaítē, "flowing hair", "mane". The specific name gnou originates from the Khoikhoi name for these animals, gnou. The common name "gnu" is also said to have originated from the Hottentot name t'gnu, which refers to the repeated calls of "ge-nu" by the bull in the mating season. The black wildebeest was first discovered in the northern part of South Africa in the 1800s.

The black wildebeest is currently included in the same genus as the blue wildebeest (Connochaetes taurinus). This has not always been the case, and at one time the latter was placed under a genus of its own, Gorgon. The black wildebeest lineage seems to have diverged from the blue wildebeest in the mid- to late Pleistocene, and became a distinct species around a million years ago. This evolution is quite recent on a geologic time scale.

Features necessary for defending a territory, such as the horns and broad-based skull of the modern black wildebeest, have been found in their fossil ancestors. The earliest known fossil remains are in sedimentary rock in Cornelia in the Free State and date back about 800,000 years. Fossils have also been reported from the Vaal River deposits, though whether or not they are as ancient as those found in Cornelia is unclear. Horns of the black wildebeest have been found in sand dunes near Hermanus in South Africa. This is far beyond the recorded range of the species and these animals may have migrated to that region from the karoo.

The black wildebeest is known to hybridise with its taxonomically close relative, the blue wildebeest. Male black wildebeest have been reported to mate with female blue wildebeest and vice versa. The differences in social behaviour and habitats have historically prevented interspecific hybridisation, but it may occur when they are both confined within the same area. The resulting offspring is usually fertile. A study of these hybrid animals at Spioenkop Dam Nature Reserve in South Africa revealed that many had disadvantageous abnormalities relating to their teeth, horns, and the wormian bones in the skull. Another study reported an increase in the size of the hybrid as compared to either of its parents. In some animals, the auditory bullae are highly deformed, and in others, the radius and ulna are fused.

Black wildebeest are sexually dimorphic, with females being shorter and slenderer than males. The head-and-body length is typically between 1.7 and 2.2 m (5 ft 7 in and 7 ft 3 in). Bulls reach about 1.11 to 1.21 m (3 ft 8 in to 4 ft 0 in) at the shoulder, while cows reach 1.06 to 1.16 m (3 ft 6 in to 3 ft 10 in). Bulls typically weigh 140 to 157 kg (309 to 346 lb) and cows 110 to 122 kg (243 to 269 lb). A distinguishing feature in both sexes is the tail, which is long and similar to that of a horse. Its bright-white colour gives this animal the vernacular name of "white-tailed gnu", and also distinguishes it from the blue wildebeest, which has a black tail. The length of the tail ranges from 80 to 100 cm (2.6 to 3.3 ft).