Saint-Hernin (French pronunciation: [sɛ̃.t‿ɛʁnɛ̃]; Breton: Sant-Hern) is a commune in the Finistère department of Brittany in north-western France.

The historian and librarian Georges Le Rider (1928-2014) was born in Saint-Hernin.

Inhabitants of Saint-Hernin are called in French Saint-Herninois.

Saint-Hernin lies on the northern slope of the Montagnes Noires (french, Black Mountains).

The canal de Nantes à Brest, which is the canalized river Hyères, forms the commune's northern border.