linguistic:voice_change
Classification

Korean is a member of the Koreanic family along with the Jeju language. Some linguists have included it in the Altaic family, but the core Altaic proposal itself has lost most of its prior support. The Khitan language has several vocabulary items similar to Korean that are not found in other Mongolian or Tungusic languages, suggesting a Korean influence on Khitan.

The hypothesis that Korean could be related to Japanese has had some supporters due to some overlap in vocabulary and similar grammatical features that have been elaborated upon by such researchers as Samuel E. Martin and Roy Andrew Miller. Sergei Starostin (1991) found about 25% of potential cognates in the Japanese–Korean 100-word Swadesh list. Some linguists concerned with the issue between Japanese and Korean, including Alexander Vovin, have argued that the indicated similarities are not due to any genetic relationship, but rather to a sprachbund effect and heavy borrowing, especially from Ancient Korean into Western Old Japanese. A good example might be Middle Korean sàm and Japanese asá, meaning " hemp ". This word seems to be a cognate, but although it is well attested in Western Old Japanese and Northern Ryukyuan languages, in Eastern Old Japanese it only occurs in compounds, and it is only present in three dialects of the Southern Ryukyuan language group. Also, the doublet wo meaning "hemp" is attested in Western Old Japanese and Southern Ryukyuan languages. It is thus plausible to assume a borrowed term. (See Classification of the Japonic languages or Comparison of Japanese and Korean for further details on a possible relationship.)

Hudson & Robbeets (2020) suggested that there are traces of a pre- Nivkh substratum in Korean. According to the hypothesis, ancestral varieties of Nivkh (also known as Amuric) were once distributed on the Korean Peninsula before the arrival of Koreanic speakers.