The Ionian Sea (Greek: Ιόνιο Πέλαγος, Greek pronunciation: [iˈonio ˈpelaɣos], Italian: Mar Ionio, Italian pronunciation: [mar ˈjɔːnjo], Albanian: Deti Jon) is an elongated embayment of the Mediterranean Sea, south of the Adriatic Sea. It is bounded by southern Italy including Calabria, Sicily, and the Salento peninsula to the west, southern Albania to the north, and west coast of Greece.

All major islands in the sea belong to Greece. They are collectively referred to as the Ionian Islands, the major ones being Corfu, Zakynthos, Kephalonia, Ithaca, and Lefkada. There are ferry routes between Patras and Igoumenitsa, Greece, and Brindisi and Ancona, Italy, that cross the east and north of the Ionian Sea, and from Piraeus westward. Calypso Deep, the deepest point in the Mediterranean at −5,267 m (−17,280 ft), is located in the Ionian Sea, at 36°34′N 21°8′E.[1][2] The sea is one of the most seismically active areas in the world.