City Readers Digital Historic Collections at the New York Society Library

Polybius (; Greek: Πολύβιος, Polýbios; c. 208 – c. 125 BC) was a Greek historian of the Hellenistic period noted for his work The Histories, which covered the period of 264–146 BC in detail. The work describes the rise of the Roman Republic to the status of dominance in the ancient Mediterranean world and includes his eyewitness account of the Sack of Carthage and Corinth in 146 BC, and the Roman annexation of mainland Greece after the Achaean War.

Polybius is important for his analysis of the mixed constitution or the separation of powers in government, which was influential on Montesquieu's The Spirit of the Laws and the framers of the United States Constitution.