Ctesias (; Ancient Greek: ?, Kt?sí?s, 5th century BC), also known as Ctesias the Cnidian or Ctesias of Cnidus, was a Greek physician and historian from the town of Cnidus in Caria, when Caria was part of the Achaemenid Empire.
Ctesias, who lived in the fifth century BC, was physician to the Achaemenid king Artaxerxes II, whom he accompanied in 401 BC on his expedition against his brother Cyrus the Younger. Ctesias was part of the entourage of King Artaxerxes at the Battle of Cunaxa (401 BC) against Cyrus the Younger and his Greek mercenaries called the Ten Thousand, and brought medical assistance to the king by treating his flesh wound. He reportedly was involved in negotiations with the Greeks after the battle, and also helped their Spartan general Clearchus before his execution at the royal court at Babylon.
Ctesias was the author of treatises on rivers, and on the Persian revenues, of an account of India entitled Indica (), and of a history of Assyria and Persia in 23 books, called Persica (?), 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 17 went down to 398 BC. Of the two histories, 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, much controversy occurred, both in ancient and modern times. Although many ancient authorities valued it highly, and used it to discredit Herodotus, a modern author writes, "(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, "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.
In the twenty-fourth and last episode of the first season of Space: 1999, "The Last War", the warship from the alien planet Betha is named Ctesias.