Clearchus or Clearch (Ancient Greek: Κλέαρχος; born in Sparta circa 450 BC - died at Babylon in 401 BC), the son of Rhamphias, was a Spartan general and mercenary, noted for his service under Cyrus the Younger. Born about the middle of the 5th century BC, Clearchus was sent with a fleet to the Hellespont in 412 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 BC).