Leviathan or The Matter, Forme and Power of a Commonwealth Ecclesiasticall and Civil, commonly referred to as Leviathan, is a book by the English philosopher Thomas Hobbes (1588–1679), published in 1651 (revised Latin edition 1668).

Its name derives from the Leviathan of the Hebrew Bible. The work concerns the structure of society and legitimate government, and is regarded as one of the earliest and most influential examples of social contract theory.

Written during the English Civil War (1642–1651), it argues for a social contract and rule by an absolute sovereign. Hobbes wrote that civil war and the brute situation of a state of nature ("the war of all against all") could be avoided only by a strong, undivided government.