Cleitarchus was a Greek historian of the late fourth and early third centuries BCE. He was the son of the Persian historian Deinon, and his floruit is placed around 310 BCE. He is associated with Alexandria in Egypt, where he taught and likely composed his work.

His major work was the History of Alexander, a multi-volume account of Alexander the Great's campaigns. The work is now lost, surviving only in fragments and testimonia cited by later authors.

Cleitarchus’s history was immensely popular in antiquity and formed a core source for the "Vulgate" tradition of Alexander historiography used by Diodorus Siculus, Curtius Rufus, and Justin. Though later historians like Arrian criticized its sensational and rhetorically embellished style, its vivid, dramatic narrative focusing on the exotic and marvelous was instrumental in shaping the legendary image of Alexander for subsequent centuries.