Saint Augustin (also known as Ianantsony; Malagasy: Anantsono) is a rural municipality in Madagascar.

It belongs to the district of Toliara II, which is a part of Atsimo-Andrefana Region.

Ex-pirate John Rivers established a pirate settlement at Saint Augustin, resupplying passing ships for a fee and trading in slaves; his trading post lasted from 1686 until his death in 1719.

It is situated south of Toliara (Tuléar) at the mouth of the Onilahy River to the Bay of Saint-Augustin and the Indian Ocean.

[5] It is necessary to take small outrigger canoes or to make the trip around the bay (250 km) since 2021.