Phylarchus was an Athenian historian active in the 3rd century BCE. His detailed knowledge of Peloponnesian affairs suggests he spent considerable time there. A contemporary of Aratus of Sicyon, his historical narrative covered the period from 272 BCE to the death of the Spartan king Cleomenes III in 219 BCE. Ancient sources like Plutarch indicate a potential bias in his favorable portrayal of Cleomenes III.

His major work was the Histories, a narrative in 28 books covering the period from Pyrrhus of Epirus’s expedition to the Peloponnese to Cleomenes III’s death. This work is now lost, surviving only in fragments cited by later authors such as Plutarch, Athenaeus, and Polybius. A second subject, The Story of Antiochus and Eumenes of Pergamum, concerning Antiochus III and Eumenes II, may have been part of his larger Histories.

Phylarchus was a significant but controversial Hellenistic historian. Polybius heavily criticized his method, accusing him of writing sensational, tragic history designed to evoke pity rather than provide a sober account. Despite this criticism, his fragments offer valuable, if partisan, information on figures like Cleomenes III and were used extensively by Plutarch.