Gelon (born c. 540 bc—died 478) was the tyrant of the cities of Gela (491–485) and Syracuse (485–478) in Sicily.
On the death of Hippocrates, the tyrant of Gela, in 491, Gelon, who had been his cavalry commander, succeeded him. Gelon early became involved in inconclusive hostilities with Carthage. In 485, taking advantage of an appeal by the gamoroi (conservative landowners) of Syracuse, who had been driven out by the people, he made himself master of that city, leaving his brother Hieron to control Gela. Under Gelon, Syracuse grew rapidly in population and power. Mercenaries were recruited widely, and a strong fleet was built up. Gelon conquered the nearby Sicilian cities of Euboea and Megara Hyblaea (c. 483), selling their common people into slavery and bringing their oligarchs to Syracuse. He controlled the Greek and Sicel communities of east Sicily and became linked by marriage with Theron, tyrant of Acragas (later the Roman Agrigentum, modern Agrigento). When the Carthaginians invaded Sicily in 480, Theron appealed to Gelon, who was primarily responsible for the decisive Greek victory of Himera.