Luc Ferrandez is taking over Projet Montreal on an interim basis.
The party's leader, Richard Bergeron, officially stepped down on Monday, almost a year after first announcing that he would be leaving municipal politics.
In Bergeron's third attempt to be elected mayor of Montreal he came in third, following Denis Coderre and political newcomer Melanie Joly.
Ferrandez, who is also mayor of the Plateau-Mont Royal borough, will become the leader of the opposition at Montreal City Hall.
Bergeron will continue to serve as councillor of the Saint Jacques district in downtown Montreal for the rest of his term.
