Fournier takes over from long-time leader Jean Charest, who fell in his own riding last week as Quebec voters reduced his party's majority government to Official Opposition standing.
The Liberals held on to 50 seats, more than pollsters predicted. But it's the Parti Québécois who will be given the opportunity to form the government for the first time since 2003, when the Liberals took power.
Charest announced his resignation a day after the election results came in.
Charest left his last caucus meeting Wednesday morning with a smile, cracking jokes with reporters before announcing Fournier's appointment, which was unamiously approved.
Flanked by Liberal caucus members, Fournier told reporters the Liberal Party will present a strong opposition and maintain its focus on the province's economic health and security.
"We'll be there to collaborate with the government on its measures about job creation and economy, but we'll be there also to oppose them if they ever want to try some [initiatives] bringing us to some scheme of separation," he said.
He said Quebecers were "lucky" to be led by Charest, and the province made progress under his leadership. "The determination he had, the charisma he had, the way he can build a team."
'Wise in his choices'
Charest replied that feels the party is "in good hands" with Fournier at the helm.
"He is chosen because of his great qualities that are very well known, his leadership abilities, and the fact that he also has the experience and is wise in his choices," the departing premier said.
Fournier worked as a lawyer before winning his seat in Châteauguay in the 1994 provincial election. He retired from politics in 2009 before returning to run in a byelection in the Montreal riding of St-Laurent in 2010.
He was appointed justice minister in August 2010 and was re-elected in his riding in the Sept. 4.
Fournier has already said he is not interested in running for the permanent leadership post.
The Liberal Party executive committee will meet in the coming weeks to lay out the rules for the leadership race.
The new Parti Québécois government will be sworn in next Wednesday.