Located in France, to the west of Brittany and at the southern entrance to the English Channel, the bay of Morlaix (sometimes referred to as a harbour), is a vast indentation in the northern coast of Finistère between the Léon and the Trégor, in which the Morlaix and Penzé rivers end.

The bay can be globally interpreted as a tectonic ditch with a submeridian orientation, characterized by the placement of Hercynian granites.