(Greek Åñåôñéá) was a city of Ancient Greece, located on the western coast of the island of Euboea (modern Evvoia or Evia), facing the coast of Attica across the narrow Euboian Gulf. There is a modern Greek town of the same name on the ancient site.

Eretria was listed by Homer as one of the Greek cities which sent ships to the Trojan War. In the 8th century BC, Eretria and her near neighbour and rival, Chalcis, were both powerful and prosperous trading cities, and the Eretrians controlled the Aegean islands of Andros, Tenos and Ceos. They also held lands in Boeotia on the Greek mainland.

At the end of the 8th century, however, Eretria and Chalcis fought a prolonged war known (mainly from the account in Thucydides) as the Lelantine War. Little is known of the details of this war, but it is clear that Eretria was defeated. She lost her lands in Boiotia and her Aegean dependencies. Neither Eretria nor Chalcis ever again counted for much in Greek politics.

As a result of this defeat, Eretria turned to colonisation. She planted colonies in the northern Aegean, on the coast of Macedon, and also in Italy and Sicily.

The Eretreans were Ionians, and were thus natural allies of Athens. When the Ionian Greeks in Asia Minor rebelled against Persia in 499, Eretria joined Athens in sending aid to the rebels. As a result, Darius made a point of punishing Eretria during his invasion of Greece. In 490 the city was sacked and burned by the Persians.

During the 5th century the...