The Missolonghi Lagoon (Greek: Λιμνοθάλασσα Μεσολογγίου, Limnothalassa Mesolongiou) is a shallow lagoon located in the south of Aetolia-Acarnania, Western Greece.

It is connected with the Gulf of Patras, a bay of the Ionian Sea.

It has a triangular shape and its width varies between approximately 15 to 20 km.

Near the coast, the depth is only 10 cm and it is a swampy area.

The island of Aitoliko or sometimes the peninsula extended by 15 km and 200 m making it the longest extension, the eastern half became a canal and it resembles a dike.