ancient and modern settlement, nomós (department) of Arkadhía, of the Peloponnese, Greece, just northwest of which lay an ancient city of the same name at 1,400 feet (427 m) above sea level on the Akhíllion plain. Spreading extensively on both banks of the Helisson (Elísson) River just above its junction with the Alpheus (Alfiós), the ancient community (the name means “large city,” or “great city”) was founded on a grandiose scale (371–368 BC) by Epaminondas of Thebes as the seat of the Arcadian League and as a bastion for the southern Arcadians' containment of Sparta. Megalopolis on the Helisson was populated by the wholesale transfer of inhabitants from 40 local villages and by contingents from Tegéa, Mantineia, and other locations. Encompassed by strong walls, the city reached about 5.5 miles (9 km) in circumference; its territory, extending 24 miles (39 km) northward, was the greatest of any city-state in Arcadia. Spartan attempts to take the city, which had been weakened by the failure of the Arcadian League, were foiled in 353 and 331—as well as after 234, when Megalopolis joined the Achaean League. In 223, however, Cleomenes III of Sparta plundered it, and with the coming of Rome in 146, the city declined rapidly; in the 2nd century AD the Greek traveler Pausanias noted that it was a heap of ruins. The only great historical figures it produced were the Achaean soldier-statesman Philopoemen (c. 252–182 BC) and the historian Polybius (c. 200–after 118 BC).

Excavation by the British School at Athens in 1890–93 revealed municipal buildings north of the river that were grouped around a square agora; buildings to the south of the river included the main federal buildings, a theatre that was the largest in ancient Greece, and an adjoining Thersilion, or assembly hall.

Megalopolis is at the centre of a rich lignite-bearing region that has been exploited since the early 1970s to fuel several thermal-power stations. Pop. (1981) 4,875.