Amarynthos (Greek: Αμάρυνθος, [aˈmarinθos], also called Βάθεια Váthia), is a coastal town and a former municipality in Euboea, Greece. Since the 2011 local government reform it is part of the municipality Eretria, of which it is a municipal unit. In 2001 its population was 4,141 for the town and 7,356 for the municipal unit. The Amarynthos is 8 km east of Eretria, 27 km southeast of Chalcis, 63 km northwest of Karystos and 10 km north of Kalamos, across the gulf. The Greek National Road 44 (Thebes - Chalcis - Karystos) passes through the town.
The area of Amarynthos, along with the region of Eretria, has a long and rich history. Many scientists place Eretria of the Mycenaean period in the location of the present town of Amarynthos. The archaeological artifacts testify a permanent settlement in this location since the Late Neolithic period, around 3000 BC, which was one of the most important Helladic settlements and a significant port on the island of Euboea, during the Early Bronze Age. It had built its own trade with the Aegean Islands, concluded from the findings of Cycladic art in the region.