混合政体学说（Mixed Government）始见于柏拉图的《理想国》〈法律篇〉。柏拉图将政体分为两大类，一种是君主制，一种是民主制。大多数国家都是这两种政体的不同程度的结合，是为混合政体。

柏拉图认为混合政体综合了君主政体的智能和德性，以及民主政体的自由，是最好、最稳定的政体。这个理论被认为是近代分权式政府的理论先驱。

亚里士多德也在《政治学》里也将政体分为寡头制、民主制、混合政体、僭主政体（专政）四种。

在中文里，混合政体或混合政府多指Mixed government，而混合体制和混合政权则多指Hybrid regime。

Mixed government, also known as a mixed constitution, defines a constitution in which the form of government is a combination of democracy, aristocracy, and monarchy, making impossible their respective degenerations (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 developed within the Roman Constitution. It is commonly treated as an antecedent of separation of powers because in such a system some issues are decided by many (democracy), some other issues by few (aristocracy), and some other issues by a single person (monarchy). Unlike classical democracies, aristocracies, and monarchies, a mixed government is ruled by elected citizens rather than inherited or sorted (at the Greco-Roman time, sortition was conventionally regarded as the principal characteristic of Classical democracy).