Arcachon Basin or alternatively Arcachon Bay (French: Bassin d'Arcachon, French pronunciation: [basɛ̃ daʁkaʃɔ̃]; Gascon: Laca d'Arcaishon) is a bay of the Atlantic Ocean on the southwest coast of France, situated in Pays de Buch between the Côte d'Argent and the Côte des Landes, in the region of Aquitaine. The bay covers an area of 150 square kilometres (60 sq mi) at high tide and 40 km² (15 sq mi) at low tide. Some of its geological features are natural preservation areas.

The general shape of the Bassin d'Arcachon is that of an equilateral triangle pointing north, the southwest corner of which is open to the sea, between Cap Ferret and the town of Arcachon (more specifically, one of the suburbs of La Teste-de-Buch, Pyla-sur-Mer), through a three-kilometre-long (2 mi) narrow channel (Les Passes). On the north shore is the town of Arès, then Andernos-les-Bains on the northeast. Just south of the entrance is the Dune of Pilat. Nearly in the middle of the bay is a very particular island: L'île aux Oiseaux (Isle of the Birds).