brother-in-law of Dionysius I, tyrant of Syracuse, in Sicily; Dion was master of Syracuse intermittently between 357 and 354.

When Dionysius II, who was weak and inexperienced, succeeded his father in 367, Dion assumed control and persuaded Plato, whose friendship he had acquired, to train the new tyrant in the practical application of his philosophical principles. The experiment failed and Dion was banished, but in 357, assembling a force of 1,500 mercenaries at Zacynthus, he sailed to Sicily and was received with demonstrations of joy. After a short period of rule he was again banished and again recalled. In 354 he was assassinated. He was included in Plutarch's Lives.