The 54-year-old Charest was found near Highway 195, just south of Matane, about 400 kilometres northeast of Quebec City.
Police say their investigation leads them to believe she was not the victim of foul play.
A coroner will perform an autopsy to confirm the exact cause of death.
Charest, who was not related to former Liberal premier Jean Charest, was a member of the legislature from 2003 to 2007 before being defeated by a Parti Quebecois candidate.
She also had two unsuccessful runs as a candidate for the federal Liberals, in 2008 and 2011, in the riding of Haute-Gaspesie—La Mitis—Matane—Matapedia.