Petite mer de Gâvres, Protected lagoon in Morbihan, France.

The Petite Mer de Gâvres is a protected lagoon in Brittany that spans several square kilometers and connects to the Bay of Lorient through natural water channels. The area comprises shallow waters, wetlands, and extensive shoreline zones that provide diverse habitats for plants and animals.

The lagoon owes its existence to the Gâvres tombolo, a dune belt that formed during the Flandrian transgression about 10,000 years ago and joined the former island of Gâvres to the mainland. In October 2018 a biotope protection zone was created to safeguard the seabed at the eastern end of the lagoon.

The lagoon is a recognised wintering and stopover site for waterbirds: of the 53 species recorded in the Gâvres/Quiberon Natura 2000 area, about thirty use the marsh during part or all of the year. It holds international importance for the brent goose, the greater ringed plover and the sanderling, and national importance for seven species including the shelduck, oystercatcher and bar-tailed godwit. The Île de Kerner houses a museum on the lagoon's natural environment and history.

The best time to visit is during low tide when water levels drop and more shoreline becomes visible. Sturdy footwear is recommended since paths cross through wet and muddy sections.

The lagoon's two estran islands, Île de Kerners and Île aux Pins, belong to Riantec; the Île aux Pins is reachable on foot at low tide, while the Île de Kerners is linked by road to the Kerner district. The Île de Kerner also holds a museum on the lagoon's natural environment and history.