Sometime before January 30, 2019, a by-election will be announced for the seat in the House of Commons representing Outremont in the province of Quebec.

On August 3, the Chief Electoral Officer of Canada, Stéphane Perrault, received official notice from the Speaker of the House of Commons that the seat for Outremont (Quebec) became vacant following the resignation of Tom Mulcair, who left office to teach at the at Universite de Montreal and to become a regularly scheduled political commentator at CJAD radio and CTV news.

Under the law, the by-election date must be announced between August 14, 2018, and January 30, 2019, and will signal the start of the by-election period. According to Elections Canada, the earliest date the by-election can be held is September 24, 2018.