Clearchus (450 B.C. to 401 B.C.)was a Spartan general. During the latter part of the Peloponnesian War he was active in the Spartan campaigns in the Hellespont (battles of Cyzicus, 410, and Arginusae, 406). He served as governor of Byzantium but was ousted by the people for his tyrannous rule.
After the war he was nevertheless sent back to settle political disputes and to defend the city from Thracian attacks. However, when he ignored the recall which overtook him before his arrival, he was condemned to death
Clearchus then took refuge with Cyrus the Younger, who was planning to overthrow his brother, the Persian king Artaxerxes II. He collected for Cyrus a large force of Greek mercenaries and marched with him in 401 BC.
After the defeat and death of Cyrus at Cunaxa, Clearchus and the other Greek generals were taken prisoner and put to death through the treachery of Tissaphernes.