An unclassified language is a language whose genetic affiliation to other languages has not been established. Languages can be unclassified for a variety of reasons, mostly due to a lack of reliable data but sometimes due to the confounding influence of language contact, if different layers of its vocabulary or morphology point in different directions and it is not clear which represents the ancestral form of the language. Some poorly known extinct languages, such as Gutian and Cacán, are simply unclassifiable, and it is unlikely the situation will ever change.

A supposedly unclassified language may turn out not to be a language at all, or even a distinct dialect, but merely a family, tribal or village name, or an alternative name for a people or language that is classified.

If a language's genetic relationship has not been established after significant documentation of the language and comparison with other languages and families, as in the case of Basque in Europe, it is considered a language isolate – that is, it is classified as a language family of its own. An 'unclassified' language therefore is one which may still turn out to belong to an established family once better data is available or more thorough comparative research is done. Extinct unclassified languages for which little evidence has been preserved are likely to remain in limbo indefinitely, unless lost documents or a surviving speaking population are discovered.