Mario Dumont is stepping down as leader of the Action democratique du Quebec on Mar. 6.
In a letter to the party president, Dumont also says he will resign his provincial seat in Riviere-du-Loup at the same time.
Dumont, 38, has led the ADQ since 1994.
"I want to express to you my admiration for all ADQ members and the high esteem in which I hold them..... for their strength of conviction and their love of Quebec," Dumont said in the letter.
"I wish you the best of luck in the future."
Dumont had said he would eventually step down after the ADQ suffered a crushing defeat in the provincial election last December.
The ADQ won just seven seats after Dumont guided the right-wing party from relative obscurity to official Opposition status in 2007.
The ADQ will now decide when to hold a leadership race to replace Dumont.