In the field of dialectology, a diasystem or polylectal grammar, is a linguistic analysis set up to encode or represent a range of related varieties in a way that displays their structural differences.

The term "diasystem" was coined by linguist and dialectologist Uriel Weinreich in a 1954 paper as part of an initiative in exploring how to extend advances in structuralist linguistic theory to dialectology to explain linguistic variation across dialects. Weinreich's paper inspired research in the late 1950s to test the proposal. However, the investigations soon showed it to be generally untenable, at least under structuralist theory. With the advent of generative theory in the 1960s, researchers tried applying a generative approach in developing diasystemic explanations; this also fell short.