Saint-Pol-de-Léon (Kastell-Paol) is a commune in the Finistère department in Bretagne in northwestern France.
It is famous for its 13th century cathedral on the site of the original founded by Saint Paul Aurelian in the 6th century. It was also the scene of a battle during the Breton War of Succession, where the Montfortists and their English allies defeated an army led by Charles of Blois.
As of the census of 1999, the town has a population of .