Jeju, often called Jejueo or Jejuan in English-language scholarship, is a Koreanic language originally from Jeju Island, South Korea. It is not mutually intelligible with mainland Korean dialects. While it was historically considered a divergent Jeju dialect of the Korean language, it is increasingly referred to as a separate language in its own right. It is declining in usage, and was classified by UNESCO in 2010 as critically endangered, the highest level of language endangerment possible. Revitalization efforts are ongoing.

Korean is the native language for about 81 million people, mostly of Korean descent. It is the official and national language of both North Korea and South Korea. The language has notable differences in each of the Koreas, in part owing to different official standardizations of the language. They are still largely mutually intelligible, however. South Korean newspaper Daily NK has claimed North Korea criminalizes the use of the South's standard language with the death penalty, and South Korean education and media often portray the North's language as alien and uncomfortable.