a tyrant of Heraclea in Pontus, who was killed by Chion and Leonidas, Plato's pupils, during the celebration of the festivals of Bacchus. He had enjoyed the sovereign power for twelve years.

Clearchus was also the name of a Lacedaemonian who was sent to gain over the Byzantines to the Lacedaemonian interest. Being soon afterwards recalled, he refused to obey, and fled to Cyrus the Younger, who made him captain of 13,000 Greeks, whom he had hired to fight against his brother. He obtained a victory over Artaxerxes, who was so enraged at the defeat that when Clearchus fell into his hands by the treachery of Tissaphernes, the king immediately put him to death.