Saint-Marc-du-Lac-Long () is a parish municipality in the Canadian province of Quebec, located in the Témiscouata Regional County Municipality. It has a total area of 155.9 square kilometres, with a total land area of 148.64 square kilometres.
In the 2021 Census of Population conducted by Statistics Canada, Saint-Marc-du-Lac-Long had a population of living in of its total private dwellings, a change of from its 2016 population of . With a land area of , it had a population density of in 2021.