Antirrio (Greek: Αντίρριο, pronounced [aˈndirio], Latin: Antirrhium) is a town and a former municipality in Aetolia-Acarnania, West Greece, Greece. Since the 2011 local government reform it is part of the municipality Nafpaktia, of which it is a municipal unit.

It is situated on the north side of the narrow (2.4 km) Strait of Rio, which separates the Gulf of Patras in the west from the Gulf of Corinth in the east. Since August 2004 the Rio–Antirrio bridge connects northwestern Greece with the Peloponnese. The name Antirrio means “opposite Rio“.

Antirrio is situated at the western entrance of the Gulf of Corinth. It is 3 km northwest of Rio, 10 km north of Patras and 9 km southwest of Naupactus. 8 km to the northwest is the 1,039 m high mountain Klokova[4] (also known as Paliovouna), rising steeply from the Gulf of Patras coast. The Rio–Antirrio bridge, carrying Motorway 5, connects it with the Peloponnese peninsula. Greek National Road 5 connects it with Missolonghi and northwestern Greece, Greek National Road 48 with Naupactus and Livadeia.

The Port of Antirrio was an important ferry port for traffic from the Peloponnese to northwestern Greece, but its significance was decreased when the new Rio-Antirrio bridge was opened. Adjacent to the port is a lighthouse and the fortress of Antirrio.