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 àBrest, 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.
In French the inhabitants of Châteaulin are known as Châteaulinois.
In 2008 1.96% of primary-school children attended bilingual schools.
Châteaulin's economy is based on food processing (salmon and poultry) and, to an ever-greater extent, tourism. The Gendarmerie school on the outskirts of the town provides for a large number of families and the many recruits passing through the school provide the economic backbone of the town's night-life.
Châteaulin is twinned with: