Eretria (Greek: Ερέτρια) was a polis in Ancient Greece, locate on the western coast of the island of Euboea (modern Evvoia or Evia), south of Chalcis, facing the coast of Attica across the narrow Euboian Gulf. Eretria was an important Greek polis in the 6th/5th century BC. It however lost its importance already in antiquity. There is a modern Greek town of the same name on the ancient site.