ancient Greek city situated on the Chalcidice Peninsula of northwestern Greece. It lay about 1.5 miles (2.5 km) inland from the Gulf of Torone of the Adriatic Sea. A Thracian people called the Bottiaeans inhabited Olynthus until 479 BC, when Persian forces killed them and handed the town over to local Greeks from Chalcidice. Though dominated for a time thereafter by Athens, Olynthus revolted against the latter in 424 and was subsequently able to maintain its independence. Olynthus became the chief Greek city west of the Strymon (modern Struma) River, and in 432 it founded and became the chief city of the Chalcidian League, a confederation of the Greek cities of the Chalcidice Peninsula. By 382 the league's power had aroused the hostility of Sparta, which, after three years of fighting, defeated Olynthus and disbanded the league in 379. But after the defeat of Sparta by Thebes in 371, Olynthus reestablished the league and was able to attain even greater wealth and power than before. When war broke out between Philip II of Macedon and Athens (357), Olynthus initially allied itself with Philip. Fearing the latter's increasing power, however, Olynthus shifted its allegiance to Athens. Philip's consequent threats against Olynthus prompted Demosthenes to deliver three great speeches (the “Olynthiacs”) urging Athens to aid Olynthus. But the Athenians did nothing, and Philip razed Olynthus in 348.

Excavations by the American School of Classical Studies at Athens revealed the grid plan of the ancient town and provided material for study of the relations between Classical and Hellenistic Greek art. The site is occupied by the modern town of Ólinthos.