Ephorus of Cyme (/ˈɛfərəs/; Greek: Ἔφορος Ephoros; c. 400 – 330 BC), often named in conjunction with his birthplace Cyme, Aeolia, was an ancient Greek historian. Information on his biography is limited; he was the father of Demophilus, who followed in his footsteps as a historian, and to Plutarch's claim that Ephorus declined Alexander the Great's offer to join him on his Persian campaign as the official historiographer. Together with the historian Theopompus, he was a pupil of Isocrates, in rhetoric. But he does not seem to have made much progress in the art, and it is said to have been at the suggestion of Isocrates himself that he took up literary composition and the study of history.
The fruit of his labours was a set of 29 books, his universal history. The whole work, edited by his son Demophilus - who added a 30th book - contained a summary description of the , along with other narratives from the days of the Heraclids up until the taking of Perinthus in 340 BC by Philip of Macedon, covering a time span of more than seven hundred years. According to Polybius, Ephorus was the first historian ever to author a universal history. For each of the 29 separate books Ephorus wrote a prooimion. The work was probably simply named Historiai, and followed a thematic, rather than a strictly chronological order in its narrative. Diodorus Siculus was largely responsible for preserving this work for posterity, by copying large parts of his writings. Book 30, covering the years 356-340 BC, was added by Demophilus quite probably after his death. The excerpts of their writings in Diodorus constitute the only continuous narrative on the history of Greece between 480-340 BC.