The Baie de Douarnenez is a bay in Finistère, France, between the Crozon peninsula to the north and the cap Sizun to the south. It is formed of a vast semi-circular basin over 16 km wide and 20 km deep. Although half-closed-off to the west by cap de la Chèvre, it opens out again to a width of 9 km on the side of the mer d'Iroise. To the south-east is to be found the sardine port of Douarnenez which gives the bay its name, the only settlement of importance on the banks of the bay. The bathing station at Morgat is located to the north, on the Crozon peninsula.
Also in this bay is the town of Ys, engulfed in a famous legend.