Maria Mourani was acclaimed by the party at a reception Wednesday evening in front of about 200 NDP supporters.
The popular MP is tasked with flipping the seat for the NDP as the party looks to defend the 10 ridings on the island of Montreal it won from the Bloc and the Liberals in 2011.
The four other candidates who wanted a spot on the NDP ticket dropped out of the race when they heard Mourani wanted to switch teams, a party official told The Canadian Press.
African-born Mourani was the first Lebanese woman elected to Parliament.
She had been sitting as an Independent MP since 2013 after being kicked out of the Bloc for her outspoken criticism of the Parti Quebecois' proposed secularism charter.
The riding has a high percentage of immigrants, many of whom are from Algeria, Lebanon and Morocco — some of the ethnicities she felt were particularly targeted by the PQ's charter.
While Mourani now says she is a federalist, she admitted she didn't know how her former sovereignty stance would affect the riding's NDP voters.
"It's going to be interesting," she said after making her speech to the party faithful.
Mourani said when she left the Bloc in 2013, "about half" of her supporters stuck with her, despite her abandonment of sovereigntist principles.
"When I was in the Bloc I had federalists who voted for me," she said, suggesting that some sovereigntists will vote for her in the fall. "Some people vote for the candidate."
NDP Leader Thomas Mulcair said at Mourani's acclamation that he wasn't concerned his new candidate's past politics will hurt the party.
He said Mourani has chosen to adopt the NDP's "beautiful and hopeful" vision for the country.
Mourani won the past three elections in Ahuntsic for the Bloc, however, her wins were by slim margins.
Since 1993 the riding has switched back and forth between the Bloc and the Liberals, who are both looking to steal back many of the Quebec ridings the NDP won from them in 2011.
Ahuntsic's boundaries have changed to include a chunk of the neighbouring riding of Saint-Laurent-Cartierville, which is represented by the former Liberal leader Stephane Dion, who has won each of his elections comfortably since 1996.
Mourani said she isn't worried about the boundary changes.
"There is no such thing as 'Liberal territory,'" she said.
Neither the Liberals nor the Bloc have selected candidates for the Ahuntsic-Cartierville riding.