Eretria (/əˈriːtriə/; Greek: Ερέτρια, Erétria, Ancient Greek: Ἐρέτρια, Erétria, literally 'city of the rowers') is a town in Euboea, Greece, facing the coast of Attica across the narrow South Euboean Gulf. It was an important Greek polis in the 6th and 5th century BC, mentioned by many famous writers and actively involved in significant historical events.

Excavations of the ancient city began in the 1890s and have been conducted since 1964 by the Greek Archaeological Service (11th Ephorate of Antiquities) and the Swiss School of Archaeology in Greece.

The first evidence for human activity in the area of Eretria are pottery shards and stone artifacts from the late Neolithic period (3500–3000 BC) found on the Acropolis as well as in the plain. No permanent structures have yet been found. It is therefore unclear whether a permanent settlement existed at that time.

The first known settlement from the Early Helladic period (3000–2000 BC) was located on the plain. A granary and several other buildings, as well as a pottery kiln, have been found so far. This settlement was moved to the top of the Acropolis in the Middle Helladic period (2000–1600 BC) because the plain was flooded by the nearby lagoon. In the Late Helladic period (1600–1100 BC), the population dwindled and the remains found so far have been interpreted as an observation post. The site was abandoned during the Greek Dark Ages.

The oldest archaeological finds date the foundation of the city to the Greek Dark Ages.

The earliest surviving mention of Eretria was by Homer (Iliad 2.537), who listed Eretria 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. Eretria controlled the Aegean islands of Andros, Tenos and Ceos. They also held territory in Boeotia on the Greek mainland. Eretria was also involved in the Greek colonisation and founded the colonies of Pithekoussai and Cumae in Italy together with Chalcis.

At the end of the 8th century BC, however, Eretria and Chalcis fought a prolonged war (known mainly from the account in Thucydides as the Lelantine War) for control of the fertile Lelantine plain. Little is known of the details of this war, but it is clear that Eretria was defeated. The city was destroyed and Eretria lost her lands in Boeotia 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, in Italy, and in Sicily.

It became an important city in the 6th/5th century BC mentioned by many famous writers and actively involved in significant historical events. The Eretrians were Ionians and were thus natural allies of Athens. When the Ionian Greeks in Asia Minor rebelled against Persia in 499 BC, Eretria joined Athens in sending aid to the rebels, because Miletus had supported Eretria in the Lelantine War. The rebels burned Sardis, but were defeated and the Eretrian general Eualcides was killed. Darius made a point of punishing Eretria during his invasion of Greece. In 490 BC the city was sacked and burned by the Persians under the admiral Datis. In retribution for the stout resistance, the victors killed all the male citizens and deported women and children barefoot to Arderikka in Susiana, Persia, forcing them into slavery. The Persians also destroyed the great temple of Apollo, built around 510 BC; parts of a pediment were found in 1900, including the torso of a statue of Athena.