Thessaly (thĕs´əlē), largest ancient region of Greece in N central Greece. It corresponded roughly to the present-day nomes of Larissa and Tríkkala, which form part of the modern region known as Thessaly. Ancient Thessaly was almost completely walled in by mountains, including Pindus, Ossa, and Othrys (now Othrís), and the plains were extremely fertile. Civilization dates from prehistoric times. Before 1000 BC a tribe called the Thessalians entered the area from the northwest. The chief Thessalian cities, Larissa, Crannon, and Pherae, were oligarchical. The great families were the Aleuadae (at Larissa) and the Scopadae (at Crannon). The Thessalians were powerful in the 6th cent. BC, partly through their control of the Amphictyonic League (see amphictyony). Conflict between the oligarchies, however, contributed to Thessaly's decline. Jason, the tyrant of Pherae, succeeded (374 BC) in uniting Thessaly, which again became a force in Greece, but it did not remain powerful for long and was subjugated (344 BC) by Philip II of Macedon. Under the Roman emperors Thessaly was joined to Macedonia, but after the death of Constantine the Great it became a separate province. It passed (1355) to the Turks and was ceded to Greece in 1881.
See A. J. B. Wace, Prehistoric Thessaly (1912); H. D. Hansen, Early Civilization in Thessaly (1933); and H. D. Westlake, Thessaly in the Fourth Century BC (1935, repr. 1969).