Lac Saint-Jean is a large, relatively shallow lake in south-central Quebec, Canada, in the Laurentian Highlands. It is situated 206 kilometres north of the Saint Lawrence River, into which it drains via the Saguenay River.
The lake is fed by dozens of small rivers with towns that dot its shores including Alma, Dolbeau-Mistassini, Roberval, Normandin, and Saint-Félicien. The lake was initially named Piékouagami (Flat Lake) by the Kakouchak Innu who lived on its shores. It was given its French name after Jean Dequen, a Jesuit missionary who in 1647 was the first European to reach its shores.
It is the site of the annual FINA 10 km Marathon Swimming World Cup race and the 32 km Traversée internationale du lac St-Jean, considered by many to be the top professional marathon swimming competition in the world.