Saint-Perreux (; ) is a commune in the Morbihan department of Brittany in north-western France. It was named after the Welsh monk, Saint Petroc.
The river Arz forms most of the commune's western and south-western borders, then flows (at Saint-Jean-la-Poterie) into the Oust, which forms the commune's eastern border.
Inhabitants of Saint-Perreux are called in French Perrusiens or Perreusiens.