Greek historian and influential politician of the Achaean League.

Polybius (c. 203 BCE - 120 BCE, Greek Πολυβιος) was a Greek historian of the Mediterranean world famous for his book called The Histories or The Rise of the Roman Empire, covering in detail the period of 220 BCE to 146 BCE. He is also renowned for his political ideas of political balance in the government, which was later used in the drafting of the United States Constitution.

Polybius is a lunar impact crater in the southeast part of the Moon. It is located to the south-southeast of the larger Catharina crater, in the area framed by the Rupes Altai scarp. Some distance to the northeast is the Mare Nectaris, with the Beaumont and Fracastorius flooded craters.

Tiberius Claudius Polybius (fl. 1st century AD) was a freedman of Claudius who was elevated to the secretariat during his reign. He assisted Claudius in his literary, judicial, historical pursuits as a researcher before the emperor's accession, and this became Polybius' official role in the imperial bureaucracy, with the title 'a studiis'. Suetonius claims that Claudius was so appreciative of the help that Polybius was allowed to walk between the consuls when on official business.

Polybius is supposedly an arcade game, thought to be similar in genre to Atari's Tempest, and the subject of an online urban legend.