(Θρασυδαῖος), tyrant of Agrigentum, was the son and successor of Theron. Already during his father's lifetime he had been appointed to the government of Himera, where, by his violent and arbitrary conduct, he alienated the minds of the citizens, so that they were on the point of breaking out into revolt. But having applied for support to Hieron of Syracuse, that ruler betrayed their application to Theron, who, in consequence, put to death the leaders of the disaffected party, and effectually re-established his authority. (Diod. 11.48.) Whether Thrasydaeus retained his position at Himera after this, we know not : but on the death of Theron he succeeded without opposition in the sovereignty of both cities. His tyrannical and violent character soon displayed itself, and rendered him as unpopular at Agrigentum as he had been at Himera. But his first object was to renew the war with Hieron, against whom he had already taken an active part during his father's lifetime. (Schol. ad Pind. Ol. 2.29.) He therefore assembled a large force of mercenaries, besides a general levy from Agrigentum and Himera, and advanced against Hieron, but was defeated after an obstinate and sanguinary struggle ; and the Agrigentines immediately took advantage of this disaster to expel him from their city. He made his escape to Greece, but was arrested at Megara, and publicly executed. (Diod. 11.53.) Diodorus assigns the whole of these events to the year B. C. 472, in which Theron died, but there are some difficulties in this chronology. (See Böckh, ad Pind. vol. iii. p. 209; and Brunet de Presle, Recherches sur les Etablissements Grecqes en Sicile, p. 145, note.)