Polls opened at 10 a.m. ET on Sunday to elect a replacement for Marie Calderone, who resigned from her position in June.
Calderone was a member of Union Montréal, the former Montreal mayor's party.
Amid the high tension at city council, the byelection is considered to be a test for the three parties.
On Tuesday, Gérald Tremblay stepped down in the wake of allegations that he had turned a blind eye to corruption and electoral misspending by his party — charges he strenuously denied.
Friday, Tremblay's right-hand man, Michael Applebaum, quit his job as chair of the city's executive committee, over the committee's refusal to back a rollback of the proposed 3.3 per cent tax hike and his claim that his colleagues had hidden a report about the inflated cost of public works projects.
Leader of Projet Montréal Richard Bergeron said "what we need now is to regain the trust of Montrealers. The byelection in Rivière-des-Prairies could be an important turn."
Louise Harel, leader of Vision Montréal, announced Saturday she would deposit a formal request for the said "hidden" document before the access to information commission.
Nino Colavecchio of Union Montréal, Cindy Leclerc of Vision Montréal and Nathalie Pierre-Antoine of Projet Montréal are running for election in the district.
Polling stations close at 8 p.m.