Flanked by PQ Leader Pauline Marois, Peladeau told a news conference he's worked for 25 years to build up Quebecor Media Inc. (TSX:QBR.B) and now wants to devote himself to public service.
Peladeau, until recently the head of the powerful conglomerate, said his dream is to help Quebec become a country.
"Quebec has all the means to succeed. We have financial resources, we have human resources, we have natural resources," Peladeau said to cheers from the party faithful in Saint-Jerome, north of Montreal, where he will run for a seat in the April 7 election.
"We've got everything (we need) for a country to be alive and kicking."
Peladeau's decision to run for the PQ represents a major coup for Marois as the party has tried to erase doubts about its ability to manage the economy.
Marois said Peladeau will bring a valuable perspective to the PQ.
"He is a sovereigntist, he is a person that has great experience in business, and I've convinced him to work with me and my team for the progress of the economy of Quebec, and the progress of Quebec," the PQ leader said.
The Saint-Jerome riding is seen to be up for grabs after Jacques Duchesneau, well known for his anti-corruption work, decided not to run again for Francois Legault's Coalition party.
Peladeau rejected suggestions his entry into politics could represent a conflict of interest, given his powerful presence in the province's media landscape.
Quebecor, founded by Peladeau's father, has a number of holdings in Quebec, including the tabloid newspaper Journal de Montreal, the French-language TVA television network, and Videotron.
It also owns the Sun tabloids and the Sun News Network in English Canada, a right-wing television station known for its anti-separatist stance.
Peladeau said he resigned Sunday morning as vice-chairman of the conglomerate.
While he will retain his ownership stake, Peladeau said his media outlets will be independent in their coverage and he will place his investments in a blind trust.
"All the editorial management functions have been independent in Quebecor forever, from the Journal de Montreal to TVA," he said.
Peladeau said Sun News and his other media holdings can "say whatever they want" about the PQ and the push for an independent Quebec.
"It's their own opinion," he said. "This is what the media is all about."
Rumours had been swirling for months that Peladeau would run for the PQ.
Last year he stepped down as Quebecor CEO to take a post as chairman of Hydro-Quebec, the provincial power utility.
He said Sunday he had resigned from that post as well.
Rival party leaders spent much of Sunday responding to the surprise announcement.
Liberal Leader Philippe Couillard, who was campaigning in his home riding of Roberval, tried to downplay Peladeau's arrival on the political scene.
"All candidates are welcome," he said.
"It's good to go into politics. But a candidate, no matter who he is, won't change the debate."
Legault took a different tack, saying Peladeau will be "disappointed" with his decision to run for the PQ.
Legault was a PQ minister himself before establishing the Coalition, which promises to put the debate over sovereignty on hold while focusing on the economy.
"I realized that Party Quebecois reforms on public finances and the economy are all held hostage by one subject: a referendum," he said.
Peladeau's candidacy didn't sit well with the province's powerful labour movement.
The FTQ, the province's largest union, said in a statement Sunday he's "probably one of the worst employers Quebec has ever known."
Citing Quebecor's history of labour disputes, the FTQ called the candidacy a "catastrophe" for Quebec workers.
Francoise David, spokeswoman for the left-wing Quebec solidaire, said it was further sign the PQ had lost touch with its progressive roots.
David said an elected member of Quebec solidaire will "never sit beside Pierre Karl Peladeau" in Quebec's national assembly.
- with files from Patrice Bergeron in Dolbeau-Mistassini, Melanie Marquis in Mont-Saint-Gregoire, and Catherine Gignac
Follow @benshingler on Twitter