Eupalinos (Ancient Greek: Εὐπαλῖνος) or Eupalinus of Megara was an ancient Greek engineer who built the Tunnel of Eupalinos on Samos Island in the 6th century BC.

The tunnel, presumably completed between 550 and 530 BC,^[1] is the second known tunnel in history which was excavated from both ends and the first with a methodical approach in doing so.^[2] Being also the longest tunnel of its time, the Tunnel of Eupalinos is regarded as a major feat of ancient engineering. It was constructed for the tyrant Polycrates of Samos, and was a remarkable 1,036 meters (3,399 ft) long. It brought water to the city, passing through limestone at the base of a hill; this tunnel still exists.