For the beetle genus, see Ctesias (beetle).

Ctesias (; , Ktesías), also known as Ctesias the Cnidian or Ctesias of Cnidus, was a Greek physician and historian from the town of Cnidus in Caria. Ctesias, who lived in the 5th century BC, was physician to Artaxerxes Mnemon, whom he accompanied in 401 BC on his expedition against his brother Cyrus the Younger.