Châteaulin is a commune in the Finistère department and administrative region of Brittany in north-western France. It is a sub-prefecture of the department. Châteaulin is in a valley towards the center of Finistère on the River Aulne and the Canal de Nantes à Brest, mid-way between Quimper to the south and Brest to the north. To the west the Menez-Hom hills separate it from the Crozon peninsula and the Bay of Douarnenez, which opens into the Atlantic Ocean.