Ctesias (; Ancient Greek: Κτησίας, Ktēsíās), 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.
Ctesias was the author of treatises on rivers, and on the Persian revenues, of an account of India entitled Indica (Ἰνδικά), and of a history of Assyria and Persia in 23 books, called Persica (Περσικά), written in opposition to Herodotus in the Ionic dialect, and professedly founded on the Persian Royal Archives.
The first six books covered the history of Assyria and Babylon to the foundation of the Persian empire; the remaining seventeen went down to the year 398 BC. Of the two histories, we possess abridgments by Photius, and fragments are preserved in Athenaeus, Plutarch, Nicolaus of Damascus and especially Diodorus Siculus, whose second book is mainly from Ctesias. As to the worth of the Persica there has been much controversy, both in ancient and modern times. Although many ancient authorities valued it highly, and used it to discredit Herodotus, a modern author writes that "(Ctesias's) unreliability makes Herodotus seem a model of accuracy." Ctesias's account of the Assyrian kings does not reconcile with the cuneiform evidence. The satirist Lucian thought so little of Ctesias' historical reliability that in his satirical True Story he places Ctesias on the island where the evil were punished. Lucian wrote that "The people who suffered the greatest torment were those who had told lies when they were alive and written mendacious histories; among them were Ctesias of Cnidus, Herodotus, and many others."
A record of the view that the Persians held of India, under the title Indica, it includes descriptions of god-like people, philosophers, artisans, and unquantifiable gold, among other riches and wonders. It is of value as it records the beliefs of the Persians about India. The book only remains in fragments and in reports made about the book by later authors.