Clearchus or Clearch (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 appointed by the ephors to settle the political dissensions then rife at Byzantium and to protect the city and the neighbouring Greek colonies from Thracian attacks, he made himself tyrant of Byzantium, and, when declared an outlaw and driven thence by a Spartan force, he fled to Cyrus, leaving the garrison to Helixus of Megara (see Coeratadas).
In the "expedition of the ten thousand" undertaken by Cyrus to dethrone his brother Artaxerxes Mnemon, 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 Tissaphernes, he was handed over to Artaxerxes and executed .