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. It covers an area of , and is at its deepest point. The water is reddish yellow. Its name in the Innu language is Piekuakami.

Lac-Saint-Jean (formerly Lac-Saint-Jean—Saguenay) is a federal electoral district in Quebec, Canada, that was represented in the House of Commons of Canada from 1949 to 2004, and will be represented again after the 2015 election.