Philippe Couillard is considered by many to be the front-runner in a contest that so far also features ex-cabinet ministers Raymond Bachand and Pierre Moreau.
The 55-year-old Couillard, who held the health portfolio between 2003 and 2008, made the announcement at a news conference in Montreal on Wednesday.
He said he wants the Liberals to stake a claim on issues of Quebecois identity and language — and to do it in a very different way from the Parti Quebecois.
In a scathing criticism of the PQ's identity politics, he denounced the current government's plans to create a new Quebec citizenship process based on language and to change the immigration system to favour people from certain francophone countries.
Couillard suggested the Liberal party's approach is more inclusive, and hinged on the belief that Quebec's identity is more secure if the province holds greater economic clout.
"We've always been an inclusive party," Couillard said.
"What we see here (from the PQ) is an attempt to categorize people as good Quebecers and not-as-good Quebecers. And I totally reject this from a human point of view. I cannot accept a society that would live under this type of definition.
"We are all Quebecrs — we are all proud to be part of a francophone state in North America. And this includes everyone ... be they francophone, anglophone or allophone."
Couillard also said he hopes the province eventually endorses the Canadian Constitution but he downplayed the urgency of the matter and suggested it wasn't a top priority for now.
He made his launch surrounded by numerous current and former caucus members, including onetime ministers of health, justice, transport and intergovernmental affairs in the last Liberal government.
Couillard, a former neurosurgeon, professor and health-system administrator, was considered the biggest star in the early days of the Charest era with an easygoing speaking style and quick with an answer in front of the news cameras, even as a political rookie in 2003.
But he was criticized in some quarters after his political retirement for negotiating the terms of his private-sector employment while he was still a cabinet minister.
He insists he did nothing wrong and that the controversy won't hurt him in the leadership campaign.
He defended his integrity and said he would not be getting back into politics, with all the scandals swirling in Quebec recently, if he felt he had anything to hide.
Couillard applauded the work being done by the province's ongoing corruption inquiry. He said it's been extremely useful in shedding valuable light on the mechanics of public works and its relationship to political parties.
He also suggested the matters being raised should be of interest to everyone — including people outside Quebec.
Couillard said people might be sorely mistaken if they think such corruption is only a Quebec phenomenon.