Agatharchides of Cnidus was a Greek historian and geographer of the Hellenistic period, active in Alexandria during the reigns of Ptolemy VI Philometor and Ptolemy VIII Euergetes II. Described as a Peripatetic, he served as a tutor and spent his career within the Ptolemaic court, where he accessed official archives. He likely retired to write following political upheaval under Ptolemy VIII.

His major works survive only in fragments preserved by authors like Photius, Diodorus Siculus, and Strabo. These include Events in Asia in ten books, Events in Europe in forty-nine books, and the significant geographical treatise On the Erythraean Sea in five books.

Agatharchides is a crucial source for the geography and ethnography of the Hellenistic world, particularly for the Red Sea region, Arabia, and Northeast Africa. His critical methodology and vivid narrative, especially evident in On the Erythraean Sea, made him an important authority for later geographers and historians.