Clearchus or Clearch (Ancient Greek: Κλέαρχος), the son of Rhamphias, was a Spartan general and mercenary.
Born about the middle of the 5th century BC, Clearchus was sent with a fleet to the Hellespont in 411 and became governor of Byzantium, of which town he was proxenus. His severity, however, made him unpopular, and in his absence the gates were opened to the Athenian besieging army under Alcibiades (409).
Subsequently Clearchus returned to Sparta and appealed to the ephors, asking to be given a force to settle the political dissensions then rife at Byzantium and to protect the city and the neighbouring Greek colonies from Thracian attacks. He was granted that force, but when the ephors learned that the citizens of Byzantium considered him a tyrant, they recalled him through a messenger that reached Clearchus while he was still in the Isthmus of Corinth. Clearchus ignored the messenger and proceeded to Byzantium, and thus he was instantly declared an outlaw by the ephors. He fought the Thracian tribes successfully, in the process gaining the unofficial support of the Greek cities that were thus relieved. Clearchus, counting on his successes to gain him back the Spartan ephors' good graces, was ultimately disappointed in this expectation. When Cyrus learned that a Greek force in high fighting condition was so near Asia, he sent ambassadors with money (10,000 Persian darics) asking Clearchus to help him claim the throne from his brother, the Persian Emperor Artaxerxes II Mnemon. Clearchus accepted not because of the money but because he knew that sooner or later he would have to face his fellow Spartans since he was still considered an outlaw by the ephors. He left the command of the garrison of Byzantium to Helixus of Megara (see Coeratadas).
In the "expedition of the ten thousand" undertaken by Cyrus to dethrone his brother Artaxerxes, Clearchus led the Peloponnesian delegation of the Army of the Ten Thousand, who formed the right wing of Cyrus’s army at the battle of Cunaxa (401). On Cyrus’s death Clearchus assumed the chief command and conducted the retreat until, being treacherously seized with his fellow-generals by the satrap of Sardis, Tissaphernes, he was handed over to Artaxerxes and executed.