Ephorus of Cyme was a Greek historian from Cyme in Aeolis, active in the 4th century BCE. He and Theopompus of Chios were students of the rhetorician Isocrates in Athens, an association that shaped his rhetorical and moral approach to historiography.

His major work was the Histories, a universal history in 30 books covering from the Return of the Heracleidae to the siege of Perinthus in 341 BCE. It is now lost but survives in fragments. His other lost works include On Inventions and On Style.

Ephorus pioneered universal history by creating a synchronized narrative of the Greek and non-Greek worlds. His thematic organization and clear prose made his work a standard source; Diodorus Siculus relied on him extensively. Though later criticized by Polybius, his comprehensive scope secured his influence on historiography.