Batz-sur-Mer is a commune in the Loire-Atlantique department in western France.
The commune is situated on a former island that, until around the 9th century, was separate from the mainland at Guérande and the neighboring island of Le Croisic. The territory of the commune is now part of the wild coast of Guérande Peninsula with rocky cliffs, sandy beaches along the Atlantic Ocean, and an extensive salt marshes to the northeast and east.
The town lies between the Bay of Biscay and its salt marshes and is a very Breton town of whitewashed granite houses.