Mixed government (or a mixed constitution) is a form of government that combines elements of democracy, aristocracy and monarchy, ostensibly making impossible their respective degenerations which are conceived as anarchy, oligarchy and tyranny. The idea was popularized during classical antiquity in order to describe the stability, the innovation and the success of the republic as a form of government developed under the Roman constitution.

Unlike classical democracy, aristocracy or monarchy, under a mixed government rulers are elected by citizens rather than acquiring their positions by inheritance or sortition (at the Greco-Roman time, sortition was conventionally regarded as the principal characteristic of classical democracy).

The concept of a mixed government was studied during the Renaissance and the Age of Reason by Niccolò Machiavelli, Giambattista Vico, Immanuel Kant, Thomas Hobbes and others. It was and still is a very important theory among supporters of republicanism. Various schools have described modern polities, such as the European Union and the United States, as possessing mixed constitutions.