The Dayak (/ˈdaɪ.ək/ ^ⓘ; older spelling: Dajak), Dyak, Dayuh or Dayaknese, are the Austronesian ethnic groups native of Borneo. It is a loose term for over 200 riverine and hill-dwelling ethnic groups, located principally in the central and southern interior of Borneo, each with its own dialect, customs, laws, territory, and culture, although common distinguishing traits are readily identifiable. The Dayak were animist (Kaharingan and Folk Hindus) in belief; however, since the 19th century there has been mass conversion to Christianity and to Islam.

It is commonly assumed that the name originates from the Bruneian and Melanau word for "interior people", without any reference to an exact ethnic group. Particularly, it derives from a related Kenyah word for "upstream" (compare with ethnonym Lun Dayeh). The term was adopted by Dutch and German authors as an umbrella term for any non-Muslim natives of Borneo. Thus, historically, the difference between Dayak and non-Dayak natives could be understood as a religious distinction. English writers disapproved of the classification made by the Dutch and Germans, with James Brooke preferring to use the term Dayak for only two distinct groups, the Land (Bidayuh) and Sea Dayaks (Iban).

The Dutch classification from the 19th century has since continued in Indonesia as a catch-all term for indigenous, often non-Muslim tribes on the island until today. The term gained traction in the early 1900s among rising middle class and intellectual figures (such as Hausman Baboe) from those tribes and being used as a unifying term for Dayaks in Kalimantan. In Malaysia, the term Dayak generally reserves as an almost exclusively reference to the natives of Sarawak, namely Iban (previously referred as Sea Dayaks) and Bidayuh (known as Land Dayak in the past). However, some modern interpretations of the term also include the Orang Ulu groups in Sarawak.

Dayaks do not speak just one language. Their indigenous languages belong to different subgroups of the Malayo-Polynesian languages, such as Land Dayak, Malayic, Sabahan (although the inclusion of Sabahan groups into Dayaks is not common in Malaysia), and Barito languages. Nowadays most Dayaks are bilingual, in addition to their native language, are well-versed in Indonesian and Malay, depending on their country of origin. Many of Borneo's languages are endemic (which means they are spoken nowhere else). This cultural and linguistic diversity parallels the high biodiversity and related traditional knowledge of Borneo.

It is estimated that around 170 languages and dialects are spoken on the island and some by just a few hundred people, thus posing a serious risk to the future of those languages and related heritage.

In 1954, Tjilik Riwut classified the various Dayak groups into 18 tribes throughout the island of Borneo, with 403 sub-tribes according to their respective native languages, customs, and cultures. However, he did not specify the name of the sub-tribes in his publication:

In Indonesia, the Dayak indigenous religion has been given the name Kaharingan and may be said to be a form of animism. In 1945, during the Japanese occupation, the Japanese referred to Kaharingan as the religion of the Dayak people. During the New Order in the Suharto regime in 1980, the Kaharingan is registered as a form of Hinduism in Indonesia, as the Indonesian state prior to 2017 only recognised 6 forms of religion i.e. Islam, Protestantism, Roman Catholicism, Hinduism, Buddhism, and Confucianism respectively. The integration of Kaharingan with Hinduism is not due to the similarities in the theological system, but because Kaharingan is the oldest belief in Kalimantan. Unlike the development in Indonesian Kalimantan, Kaharingan is not used as a religious designation in Malaysia, Brunei and Singapore, thus the traditional Dayak belief system is categorized as a form of folk animism or paganism outside of the Indonesian border. After wider recognition of indigenous beliefs in Indonesia in 2017, Kaharingan has been sometimes included under the umbrella term Aliran Kepercayaan, though some Kaharingan activists still demands further recognition as a separate religion outside of the umbrella term.

The practice of Kaharingan differs from group to group, but shamans, specialists in ecstatic flight to other spheres, are central to Dayak religion and serve to bring together the various realms of Heaven (Upper-world) and earth, and even Under-world, for example healing the sick by retrieving their souls which are journeying on their way to the Upper-world land of the dead, accompanying and protecting the soul of a dead person on the way to their proper place in the Upper-world, presiding over annual renewal and agricultural regeneration festivals, etc. Death rituals are most elaborate when a noble (kamang) dies. Due to the institutionalization of Kaharingan beliefs in Indonesia, Kaharingan practices in Kalimantan have been recently codified and remolded into more organized religion, such as with codification of Panaturan as scripture of Kaharingan in 1971, creation of official Kaharingan body Hindu Kaharingan Religion Council (Majelis Agama Hindu Kaharingan) in 1980, and standardization of its house of worship buildings called Balai Basarah. However, not all adherents agree with the Hindu classification; a rival body called the Indonesian Kaharingan Religion Council (Majelis Agama Kaharingan Indonesia) was formed to assert Kaharingan as an independent religion, distinct from Hinduism.