In historical linguistics, a proposed language is a language for which no direct evidence exists, most commonly the proto-language of a language family. Assumptions about proposed languages are based on the comparative method.

The Proto-Indo-European language (PIE) is a typical example of a proposed language. Although there is no direct evidence that this language ever existed, there is copious evidence for its existence in the many similarities of the Indo-European languages. A great amount of work has been put into the reconstruction of Proto-Indo-European, but there are no means of determining its success. The Dacian language, on the other hand, is not a proposed language. Although records of the language itself have not survived, there are records that indicate that there was such a language. The Roman poet Ovid, for example, learned the Dacian language after being exiled to Tomis in Dacia. He even composed poems in this language, which were unfortunately lost.