The Middle English creole hypothesis is a proposal that Middle English was a creole, which is usually defined as a language that develops during contact between two groups speaking different languages and that loses much of the grammatical elaboration of its source languages in the process. The vast differences between Old English and Middle English, and English's status as one of the least structurally elaborated of the Germanic languages, have led some historical linguists to argue that the language underwent creolisation at around the 11th century, shortly after the Norman conquest of England. Other linguists suggest that creolisation began earlier, during the Scandinavian incursions of the 9th and 10th centuries.
Much of the debate over the Middle English creole hypothesis revolves around how terms like "creole" or "creolisation" should be defined. While there does not exist a consensus that Middle English should be classified as a creole, there does exist a consensus that Old English underwent fairly radical grammatical simplification in the process of evolving into Middle English, and that this evolution was due in large part to contact with speakers from other language groups.