Megalopoli (Greek: Μεγαλόπολη) is a town in the southwestern part of the regional unit of Arcadia, southern Greece. It is located in the same site as ancient Megalopolis. When it was founded in 371 BC, it was the first large urbanization in rustic Arcadia. Its theater had a capacity of 20,000 visitors, making it one of the largest ancient Greek theaters. Megalopoli has several schools, shops, churches, hotels and other services. The population of Megalopoli in 2011 was 5,779 for the town proper.
Megalopoli is situated in a wide valley, surrounded by mountains: the Taygetus to the south, the Mainalo to the north, the Tsemperou to the southeast and the Lykaion to the west. Its elevation is 430 m above sea level. The river Alfeios flows through this valley, coming from the east and flowing to the north, passing south and west of the town. Its tributary Elissonas passes north of the town. The large lignite deposits around Megalopoli are being exploited by open-pit mining. The Megalopoli Power Plant, 3 km northwest of the town centre, has produced electricity from this lignite since 1969.
Megalopolis is known for its ancient ruins situated northwest of the town centre, on both banks of the river Elissonas. The ruins include an ancient theatre that used to hold up to 20,000 people and was 30 m (100 feet) tall. Other landmarks include the Thersileon with 67 pillars and a temple (11.5 m × 5 m, 37 feet x 11 feet). Herodotus reported the ancient belief that the Megalopolis area was a battleground of the Titanomachy. The foundation for this apparently was the presence of lignite deposits, which are prone to catch fire in summer and can smoulder and scorch the earth for weeks (Zeus is supposed to have slain the Titans with lightning bolts; see also below), coupled with the presence of fossil bones of prehistoric elephants and rhinoceroses. Herodotus informs his readers that the bones of "Titans" were exhibited in various places in the surrounding area at least since the 5th century BC.
The city was founded in 371 BC by the Theban general Epaminondas in an attempt to form a political counterweight to Sparta. It was one of the 40 places that were megále pólis (great city). Megalopolis became the seat of the Arcadian League in 370 BC, which in the 3rd century BC became the Achaean League. In 331 BC, Megalopolis was invaded by the Spartans and there was a battle with the Macedonians that came to Megalopolis' help. The Macedonians defeated the Spartans. In 223 BC, the Spartan king Cleomenes III burnt down the city but it was reinstated by Philopoemen, the general of the Achaean League. The city was abandoned during the Middle Ages. The nearby village of Sinano (Σινάνο), situated south of the ancient city, was renamed Megalopoli after Greek Independence.
The province of Megalopoli (Greek: Επαρχία Μεγαλόπολης) was one of the provinces of the Messenia Prefecture. Its territory corresponded with that of the current municipality Megalopoli, except the municipal unit Gortyna. It was abolished in 2006.