Agatharchides of Cnidus was a Greek historian and geographer of the 2nd century BCE. Born in the city of Cnidus in Asia Minor, he worked for much of his career in Alexandria, Egypt, under the Ptolemaic kings. He served as a tutor to a royal prince, though the prince's specific identity is uncertain. Agatharchides is described as a Peripatetic, meaning he was influenced by the empirical, observational tradition of Aristotle.

He wrote several major works, but they survive only through fragments quoted by later authors. His most important work was On the Erythraean Sea, a detailed geographical and ethnographical account of the lands around the Red Sea, the Indian Ocean, and the Persian Gulf. He also wrote two historical works, Events in Asia and Events in Europe, which covered the period following Alexander the Great, as well as an epitome summarizing them.

Agatharchides is a vital source for understanding the Hellenistic world's knowledge of the Red Sea region and Northeast Africa. Later historians like Diodorus Siculus and Strabo relied heavily on his descriptions of geography, tribal customs, and trade routes. According to modern scholars, his Peripatetic background is evident in his rational approach to explaining natural phenomena, though his writing also included vivid details meant to engage his readers. His work bridges the historical gap between Alexander's successors and the late Hellenistic period.