The Gulf of Euboea, Euboean Gulf, Euboic Sea or Euboic Gulf (modern Greek: Ευβοϊκός Κόλπος, Evvoïkós Kólpos) is an arm of the Aegean Sea

Aegean Sea

The Aegean Sea[p] is an elongated embayment of the Mediterranean Sea located between the southern Balkan and Anatolian peninsulas, i.e., between the mainlands of Greece and Turkey. In the north, it is connected to the Marmara Sea and Black Sea by the Dardanelles and Bosporus...

 between the island of Euboea

Euboea

Euboea is the second largest Greek island in area and population, after Crete. The narrow Euripus Strait separates it from Boeotia in mainland Greece. In general outline it is a long and narrow, seahorse-shaped island; it is about long, and varies in breadth from to...

 (northeast coastline) and the Greek mainland (southwest coastline). Trending diagonally northwest-southeast, the gulf is divided by the narrow Strait of Euripus, at the town of Chalcis

Chalcis

Chalcis or Chalkida , the chief town of the island of Euboea in Greece, is situated on the strait of the Evripos at its narrowest point. The name is preserved from antiquity and is derived from the Greek χαλκός , though there is no trace of any mines in the area...

 (Khalkís). The North Euboean Gulf is about 50 miles (80 km) long and up to 15 miles (24 km) wide, and the South Euboean Gulf is about 30 miles (48 km) long, with a maximum width of 9 miles (14 km).