His exit comes five months after he lost the Quebec Liberal leadership race to Philippe Couillard, finishing a disappointing third.
Bachand had been minister of economic development, then finance, through five turbulent years during which he implemented a stimulus program and drew the wrath of student protesters with tuition hikes.
Before his career as an elected politician, Bachand was a corporate executive, lawyer, a founding member of Oxfam Quebec, and got a doctorate at the Harvard Business School.
He had also been a prominent member of the Parti Quebecois, serving as a staffer in Rene Levesque's premier's office and holding a formal role in the pro-independence Yes side in the 1980 referendum.
The 65-year-old becomes the second Opposition Liberal MNA to quit in recent weeks, following Emmanuel Dubourg who resigned to run in a federal byelection.
Both are expected to claim severance pay.