|Primary outflows||Mediterranean Sea|
|Basin countries||Albania, Greece, and Italy|
|Islands||List of islands in the Ionian Sea|
|Settlements||Igoumenitsa, Parga, Preveza, Astakos, Patras, Kerkyra, Lefkada, Argostoli, Zakynthos, Kyparissia, Pylos, Kalamata, Himarë, Sarandë, Syracuse, Catania, Taormina, Messina, Catanzaro, Crotone, Taranto|
The Ionian Sea (Modern Greek: Ιόνιο Πέλαγος, romanized: Iónio Pélagos, IPA: [iˈoni.o ˈpelaɣos];[a] Italian: Mar Ionio or Mar Jonio, IPA: [mar ˈjɔːnjo];[b] Albanian: Deti Jon, IPA: [ˈdɛti ˈjɔn]) is an elongated bay of the Mediterranean Sea. It is connected to the Adriatic Sea to the north, and is bounded by Southern Italy, including Basilicata, Calabria, Sicily, and the Salento peninsula to the west, southern Albania (and western Apulia, Italy) to the north, and the west coast of Greece, including the Peloponnese.
All major islands in the sea, which are located in the east of the sea, belong to Greece. They are collectively named the Ionian Islands, the main ones being Corfu, Kefalonia, Zakynthos, Lefkada, and Ithaca.
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 5,109 m (16,762 ft), is in the Ionian Sea, at . The sea is one of the most seismically active areas in the world.
The name Ionian comes from the Greek word Ionion (Ἰόνιον). Its etymology is unknown. Ancient Greek writers, especially Aeschylus, linked it to the myth of Io. In ancient Greek the adjective Ionios (Ἰόνιος) was used as an epithet for the sea because Io swam across it. According to the Oxford Classical Dictionary, the name may derive from Ionians who sailed to the West, however the word for Ionians is spelled with an omega ( Ἴωνες) rather than an omicron as in the word for the Ionian Sea. There were also narratives about other eponymic legendary figures; according to one version, Ionius was a son of Adrias (eponymic for the Adriatic Sea); according to another, Ionius was a son of Dyrrhachus. When Dyrrhachus was attacked by his own brothers, Heracles, who was passing through the area, came to his aid, but in the fight the hero killed his ally's son by mistake. The body was cast into the water, and thereafter was called the Ionian Sea.
In the Cham Albanian dialect, the sea is known as "Fusha e zonjës", translated as "the lady's domain.
The International Hydrographic Organization defines the limits of the Ionian Sea as follows:
From south to north in the west, then north to south in the east:
The Sea was the location of the famous naval battle between Octavian and Marc Antony known as The Battle of Actium, a war fought in 31 BC, and is also famous for the hero from Ancient Greek mythology named Odysseus, who was from the island of Ithaca.