The Dominate was the second of the two phases of government in the ancient Roman Empire between its establishment in 27 BC and the formal date of the collapse of the Western Empire in AD 476. The word is derived from the Latin dominus, meaning master.

The first phase of Imperial government, the Principate, had ended with the Third Century Crisis of 235-275, which concluded when Diocletian established himself as Emperor. Moving the notion of the Emperor away from the republican forms of the Empire's first three centuries, Diocletian, his colleagues and his successors chose instead to openly display the naked face of Imperial power, adopting a style of government more influenced by the veneration of the Eastern potentates of ancient Egypt and Persia than by the heritage of civic collegiality amongst the governing class passed down from the days of the Roman Republic.

Historians nowadays reject the interpretation of the transition from Principate to Dominate as a clear, easily definable break (cf. Late Antiquity). Rather, they now characterise it as a much more subtle, gradual transformation, in which Diocletian's reforms of the Imperial office, while significant, are but one point on a sliding scale. Nevertheless, the distinction between two primary phases of Imperial government in Rome remains an important and useful one.