Agios Sostis is a small islet located in the western part of the lagoon of Messolonghi and owes its name to the homonymous church that stands there. Its history is of great importance. Back in 1770, during the Orlofeia, it was the battlefield where the pirate fleet of Doultiniones fought against the fleet of Messolonghi, defeating the latters, while in 1827 it stood a base for operations for Astingos against the Turks, who at that time occupied Vasiladi. Furthermore, Lord Byron had disembarked on the island of Ai Sostis in January 4th, 1824, in his way to Messolonghi.

 Moreover, there is a lighthouse on the island, which is visible from Tourlida and the lighthouse keeper's house, which is now abandoned. The church of Agios Sostis celebrates on September 7th, with worships organized by the Friends of the Lagoon Association in cooperation with the Metropolitan Church of St. Spyridon in Messolonghi as well as the Metropolitan Church of Etolia and Akarnania.