The Kingdom of Westphalia is a historical state in present-day Germany that existed from 1807-1813. While formally independent, it was a vassal state of France, ruled by Napoleon's brother Jérôme Bonaparte. It was named after Westphalia, but had little territory in common with that area.

The Kingdom of Westphalia was created in 1807 by merging territories ceded by Prussia in the Peace of Tilsit, among them the former Electorate of Hanover, with the Duchy of Brunswick-Lüneburg and the Electorate of Hesse. Its capital was Cassel. A number of social reforms were implemented in the kingdom, such as the abolishment of servitude, the right of free enterprise, and the Napoleonic code. The kingdom was dissolved after the Battle of Leipzig in 1813 and its predecessor states were reestablished.