The outgoing justice minister, who has deep ties to the federal Liberals, will assume that role until the party chooses a long-term successor to Jean Charest.
Fournier served as an aide to ex-federal Liberal leader Michael Ignatieff and has clashed with the federal government over harsher criminal sentencing and the abolition of the long-gun registry.
Charest announced that the party had unanimously approved Fournier's appointment during a caucus meeting Wednesday.
A question looming after last week's provincial election is whether the Parti Quebecois will manage to force its most controversial policies through the legislature while the Liberals are leaderless.
Fournier will recall from his days in Ottawa how the Harper Tories, even with a minority, succeeded at passing key legislation because the Liberals were fearful of provoking an election.
Fournier is warning he will not be steamrollered by the PQ. He alluded to the PQ's policies on language and promoting Quebec independence and he suggested he would work to stop them.
"All measures looking to divide Quebecers, or all measures that aim at separating Quebec from Canada, will be vigorously fought," Fournier said.
A lawyer by training, Fournier said he was assuming his new role with humility and he paid tribute to the Liberal party's legacy in shaping the history of Quebec.
The leadership post became vacant last week after Charest announced his resignation. Charest said he will leave political life once he transfers the premiership to the PQ's Pauline Marois next Wednesday.
His resignation after 14 years at the party helm was prompted by last week's election defeat.
However, his Liberals' loss was narrower than expected. Despite three terms in government, numerous ethics controversies and a split in the non-separatist vote, Charest's troops lost by only four seats and less than one percentage point of the popular vote.
The outgoing premier appeared in good spirits as he prepared for a final caucus meeting Wednesday. On his way in, Charest cracked jokes in a scrum with reporters.
When asked whether he would continue weighing in on political debates, he quipped he would "rather be a grandfather than a mother-in-law" — the latter being a pejorative term in Quebec politics that refers to former leaders who second-guess their successors. The 54-year-old is about to become a grandfather.
Last week, the premier lost his own riding for the first time ever. He said he spoke to his family after the election and they all agreed he should move on after 28 years in public life.
One of his possible successors is Finance Minister Raymond Bachand.
Bachand is the architect of the government's attempt to hike university fees and his candidacy could be popular with small-c conservatives. However, his distant past in the employ of Rene Levesque's PQ could cost him votes among the party's resolutely federalist grassroots.
The former business executive, university professor and public servant said he has received encouragement to run from his family and colleagues. He said he wants the party to have a successor in place quickly, before next year's budget, so the opposition can be ready to block the PQ's "disastrous" policies.
Another potential contender is Philippe Couillard, the popular former health minister.
A neurosurgeon and onetime hospital administrator, Couillard has been appointed by the Harper Tories to the Security Intelligence Review Committee, the body that monitors CSIS, Canada's spy agency.
Couillard has said he is considering entering the race.
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