Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus Pongo, orangutans were originally considered to be one species. In 1996, they were divided into two species: the Bornean orangutan (P. pygmaeus, with three subspecies) and the Sumatran orangutan (P. abelii); a third species, the Tapanuli orangutan (P. tapanuliensis), was identified definitively in 2017. The orangutans are the only surviving members of the subfamily Ponginae, which diverged genetically from the other hominids (gorillas, chimpanzees, and humans) between 19.3 and 15.7 million years ago.

The most arboreal of the great apes, orangutans spend most of their time in trees. They have proportionally long arms and short legs, and have reddish-brown hair covering their bodies. Adult males weigh about 75 kg (165 lb), while females weigh about 37 kg (82 lb). Dominant adult males develop distinctive cheek pads or flanges and make long calls that attract females and intimidate rivals; younger subordinate males do not and more resemble adult females. Orangutans are the most solitary of the great apes: social bonds occur primarily between mothers and their dependent offspring. Fruit is the most important component of an orangutan's diet, but they will also eat vegetation, bark, honey, insects and bird eggs. They can live over 30 years, both in the wild and in captivity.

Orangutans are among the most intelligent primates. They use a variety of sophisticated tools and construct elaborate sleeping nests each night from branches and foliage. The apes' learning abilities have been studied extensively. There may be distinctive cultures within populations. Orangutans have been featured in literature and art since at least the 18th century, particularly in works that comment on human society. Field studies of the apes were pioneered by primatologist Biruté Galdikas and they have been kept in captive facilities around the world since at least the early 19th century.

All three orangutan species are considered critically endangered. Human activities have caused severe declines in populations and ranges. Threats to wild orangutan populations include poaching (for bushmeat and retaliation for consuming crops), habitat destruction and deforestation (for palm oil cultivation and logging), and the illegal pet trade. Several conservation and rehabilitation organisations are dedicated to the survival of orangutans in the wild.

Most Western sources attribute the name "orangutan" (also written orang-utan, orang utan, orangutang, and ourang-outang) to the Malay words orang, meaning 'person', and hutan, meaning 'forest'. The Malay used the term to indicate forest-dwelling humans; the first recorded Malay use of "orang-utan" referring to the ape identifies it as a Western term. There is, however, some evidence to suggest that the term may have been used in regard to apes in premodern Old Malay.

In Western sources, the first printed attestation of the word for the apes is in Dutch physician Jacobus Bontius's 1631 Historiae naturalis et medicae Indiae orientalis. He reported that Malays claimed that the ape could talk but preferred not to "lest he be compelled to labour". The word appeared in several German-language descriptions of Indonesian zoology in the 17th century. It has been argued that the word comes specifically from the Banjarese variety of Malay, but the age of the Old Javanese sources mentioned above make Old Malay a more likely origin for the term. Cribb and colleagues (2014) suggest that Bontius's account referred not to apes (as this description was from Java where the apes were not known to be from) but to humans suffering some serious medical condition (most likely cretinism) and that his use of the word was misunderstood by Nicolaes Tulp, who was the first to use the term in a publication a decade later.

The word was first attested in English in 1693 by physician John Bulwer in the form Orang-Outang, and variants ending with -ng are found in many languages. This spelling (and pronunciation) has remained in use in English up to the present but has come to be regarded as incorrect. The loss of "h" in hutan and the shift from -ng to -n has been taken to suggest the term entered English through Portuguese. In Malay, the term was first attested in 1840, not as an indigenous name but referring to how the English called the animal. The word 'orangutan' in modern Malay and Indonesian was borrowed from English or Dutch in the 20th century—explaining the missing initial 'h' of 'hutan'.

The name of the genus, Pongo, comes from a 16th-century account by Andrew Battel, an English sailor held prisoner by the Portuguese in Angola, which describes two anthropoid "monsters" named Pongo and Engeco. He is now believed to have been describing gorillas, but in the 18th century, the terms orangutan and pongo were used for all great apes. French naturalist Bernard Germain de Lacépède used the term Pongo for the genus in 1799. Battel's "Pongo", in turn, is from the Kongo word mpongi or other cognates from the region: Lumbu pungu, Vili mpungu, or Yombi yimpungu.