Altaic (; also called Transeurasian) is a controversial proposed language family that would include the Turkic, Mongolic and Tungusic language families and possibly also the Japonic and Koreanic languages.^: 73 Speakers of these languages are currently scattered over most of Asia north of 35 °N and in some eastern parts of Europe, extending in longitude from Turkey to Japan. The group is named after the Altai mountain range in the center of Asia.

The hypothetical language family has long been rejected by most comparative linguists, although it continues to be supported by a small but stable scholarly minority. The research on their supposedly common linguistics origin has inspired various comparative studies on the folklore and mythology among the Turks, Proto-Mongols and Tungus people.

The Altaic family was first proposed in the 18th century. It was widely accepted until the 1960s and is still listed in many encyclopedias and handbooks. Since the 1950s, many comparative linguists have rejected the proposal, after supposed cognates were found not to be valid, hypothesized sound shifts were not found, and Turkic and Mongolic languages were found to be converging rather than diverging over the centuries. Opponents of the theory proposed that the similarities are due to mutual linguistic influences between the groups concerned. Modern supporters of Altaic acknowledge that many shared features are the result of contact and convergence and thus cannot be taken as evidence for a genetic relationship, but they nevertheless argue that a core of existing correspondences goes back to a common ancestor.

The original hypothesis unified only the Turkic, Mongolian, and Tungusic groups. Later proposals to include the Korean and Japanese languages into a "Macro-Altaic" family have always been controversial. The original proposal was sometimes called "Micro-Altaic" by retronymy. Most proponents of Altaic continue to support the inclusion of Korean, but fewer do for Japanese. Some proposals also included Ainuic but this is not widely accepted even among Altaicists themselves. A common ancestral Proto-Altaic language for the "Macro" family has been tentatively reconstructed by Sergei Starostin and others.

Micro-Altaic includes about 66 living languages, to which Macro-Altaic would add Korean, Jeju, Japanese, and the Ryukyuan languages, for a total of about 74 (depending on what is considered a language and what is considered a dialect). These numbers do not include earlier states of languages, such as Middle Mongol, Old Korean, or Old Japanese.