This article is about the Spartan general Clearchus. For others with this name, see Clearchus.
Clearchus (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.