A tyrant (from Greek τυραννος) is a usurper of rightful power, possessing absolute power and ruling by tyranny.

In the original Greek meaning "tyrant" carried no ethical censure, a tyrant was anyone who overturned the established government of a city-state, usually through the use of popular support, to establish himself as dictator, or the heir of such a person. Cypselus was the first tyrant of Corinth in the 7th century BC, and managed to bequeath his position to his son, Periander. Succession was seldom untroubled among the tyrants. In Athens, the title was first given to Pisistratus of Athens in 560 BC, followed by his sons, and with the subsequent growth of Athenian democracy, the title "Tyrant" took on its familiar censurious connotations. The Thirty Tyrants installed at defeated Athens in 404 BC by the Spartans were not tyrants in the usual sense.

The heyday of the tyrants was the early 6th century BC, when Cleisthenes ruled Sicyon in the Peloponnesus, and Polycrates ruled Samos. During this time, many governments in the Aegean world were overthrown. It was during this time that Persia first made inroads into Greece, as many tyrants sought Persian help against forces seeking to remove them.

Greek tyranny was in the main an outgrowth of the struggle of the popular classes against the aristocracy or priest-kings whose right to rule was sanctioned by archaic traditions and mythology. Tyrants were generally installed by popular coups, and were often popular rulers, at least in the early part of their reigns. For instance, Pisistratus was remembered for an episode (related by Aristotle but possibly fictional) in which he exempted a farmer from taxation because of the particular barrenness of his plot. Pisistratus' sons Hippias and Hipparchus, on the other hand, were overthrown, and Hipparchus was assassinated.

The tyrants of Sicily were the products of similar causes, but tyranny was prolonged by the threat of Carthaginian attack, which facilitated the rise of military leaders with the people united behind them. Such Sicilian tyrants as Gelon, Hiero I, Hiero II, Dionysius the Elder, and Dionysius the Younger maintained lavish courts and were patrons of culture.

Later ancient Greeks, as well as the Roman Republicans, were generally quite wary of anyone seeking to implement a popular coup. The struggle of one such Roman, Marcus Junius Brutus, is portrayed by Shakespeare in his play Julius Caesar.