Saint-Hernin (; ) 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 àBrest, which is the canalized river Hyères, forms the commune's northern border.