In linguistics, a diasystem is a term used in structural dialectology, to refer to a single genetic language which has two or more standard forms.

Examples are Danish language and Norwegian Bokmål, and the Dutch and Low Saxon languages. A well-known diasystem is that of the Serbian and Croatian languages, which were up until the breakup of Yugoslavia seen as two dialects of the Serbo-Croatian language, but are now referred to as separate languages because the political situation changed.