The Gulf of Corinth or the Corinthian Gulf (Greek: Κορινθιακός Κόλπος, romanized: Korinthiakós Kólpos, Greek pronunciation: [koɾinθiaˈkos ˈkolpos]) is an inlet of the Ionian Sea, separating the Peloponnese from western mainland Greece. It is bounded in the east by the Isthmus of Corinth which includes the shipping-designed Corinth Canal and in the west by the Rion Strait which widens into the shorter Gulf of Patras (part of the Ionian Sea) and whose narrowest point has been crossed since 2004 by the Rio–Antirrio bridge. The gulf is bordered by the large administrative divisions (regional units): Aetolia-Acarnania and Phocis in the north, Boeotia in the northeast, Attica in the east, Corinthia in the southeast and south and Achaea in the southwest. The tectonic movement across the gulf is comparable to parts of Iceland and Turkey, growing by 10 mm (0.39 in) per year.
In the Middle Ages, the gulf was known as the Gulf of Lepanto (the Italian form of Naupactus).