The classification of the modern Korean language is uncertain, and due to the lack of any one generally accepted theory, it is sometimes described conservatively as a language isolate.

Since the publication of the article of Ramstedt in 1926, many linguists support the hypothesis that Korean can be classified as an Altaic language, or as a relative of proto-Altaic. Korean is similar to Altaic languages in that they both lack certain grammatical elements, including number, gender, articles, fusional morphology, voice, and relative pronouns (Kim Namkil). Korean especially bears some morphological resemblance to some languages of the Eastern Turkic group, namely Sakha (Yakut). Vinokurova, a scholar of the Sakha language, noted that like in Korean, and unlike in other Turkic languages or a variety of other languages surveyed, adverbs in Sakha are derived from verbs with the help of derivational morphology; however, she did not suggest this implied any relation between the two languages.

It is also considered likely that Korean is related in some way to Japanese, since the two languages have a similar grammatical structure. Genetic relationships have been postulated both directly and indirectly, the latter either through placing both languages in the Altaic family, or by arguing for a relationship between Japanese and the Buyeo languages of Goguryeo and Baekje (see below); the proposed Baekje relationship is supported additionally by phonological similarities such as the general lack of consonant-final sounds, and by cognates such as Baekje mir, Japanese mi- "three". Furthermore, there are known cultural links between Baekje and Japan; historical evidence shows that, in addition to playing a large role in the founding and growth of Yamato Japan, many of the Baekje upper classes, as well as the artisans and merchants, fled to Japan when the kingdom fell (a theory which appeared to be endorsed by Japanese Emperor Akihito in a speech marking his 68th birthday).

Others argue, however, that the similarities are not due to any genetic relationship, but rather to a sprachbund effect. See East Asian languages for morphological features shared among languages of the East Asian sprachbund, and Japanese language classification for further details on the possible relationship.

Of the ancient languages attested in the Korean Peninsula, modern Korean is believed to be a descendent of the languages of Samhan and Silla; it is unknown whether these are related to the Buyeo languages, though many Korean scholars believe they were mutually intelligible, and the collective basis of what in the Goryeo period would merge to become Middle Korean (the language before the changes that the Seven-Year War brought) and eventually Modern Korean. The Jeju dialect preserves some archaic features that can also be found in Middle Korean, whose arae a is retained in the dialect as a distinct vowel.

There are also fringe theories proposing various other relationships; for example, a few linguists such as Homer B. Hulbert have also tried to relate Korean to the Dravidian languages through the similar syntax in both.