Pierre Duchesne resigned from Radio-Canada last month after 25 years at the broadcaster, most recently as chief correspondent at the legislature.
"I will defend without reservations the integrity of Pierre Duchesne," Marois told a news conference in Quebec City on Thursday.
"He is a man of great integrity."
Premier Jean Charest was circumspect when asked on Wednesday about Duchesne possibly running for the PQ, but said he might have things to say about his integrity if the rumour proved true.
Marois bristled at Charest's comments, saying the new candidate never showed any bias toward the PQ in his reports.
"I think if Mr. Charest has a problem with (Duchesne's) integrity, perhaps he is on shaky ground."
At least one source said the first discussions between the PQ and Duchesne took place at the beginning of the year while he was still employed by Radio-Canada.
Marois said that while Duchesne had been considered as a potential candidate, the PQ only approched him in the last several days and that she had only met him herself on Wednesday.
"When we saw he was resigning, it made our work easier," Marois said. "We had targeted him as an interesting candidate."
Duchesne will run in Borduas, a riding near Montreal that is currently held by former PQ member Pierre Curzi. Curzi quit the party in 2011 to sit as an Independent and has announced he is not running again.
Duchesne is also the author of an unauthorized biography on former Quebec premier Jacques Parizeau.
"It (the book) left the impression he might find it interesting if Quebec became sovereign one day," Marois said of Duchesne.
Marois took aim at Charest during the brief news conference, suggesting he had made several campaign-style announcements recently to influence his party's chances in the next election.
Marois said the premier's use of attack ads and efforts to bury his government's record is "profoundly sad for democracy and very cynical on the part of Mr. Charest."
There is persistent speculation that Charest will call an election early next month for the first few weeks of September. His mandate does not expire until late 2013.