Gabriel Nadeau-Dubois, who once wore the red square as a student leader in the Maple Spring, will swap it for an orange Québec Solidaire campaign button as the party's official candidate for the Montreal riding of Gouin.
Nadeau-Dubois was given the nod from the party faithful at a nomination meeting on Sunday night.
A date for that byelection has not been set.
Just over two weeks ago the 26-year-old said he wanted take a leadership role with the pro-sovereignist, left-wing party.
At the time he said, "It's the only party where I feel at home."
Gouin is the former riding of Françoise David, one of the party's founders. David vacated the seat this past January after deciding to leave politics.
Nadeau-Dubois is also seeking the role of party co-spokesperson.
Andrés Fontecilla, who holds one of the positions, announced he would be leaving the job in May to focus on winning a seat in the next provincial general election in 2018.
Manon Massé, the MNA for Sainte-Marie–Saint-Jacques, is also seeking a position as party co-spokesperson.
Membership up, possible political merger looming
Québec Solidaire said after Nadeau-Dubois announced his intention to run they saw a considerable boost in membership. According to their count, approximately 2,000 new members signed up with the party online in the days that followed.
Nadeau-Dubois said he wants to make the party younger, more diverse and expand it beyond its base in the Montreal area.
Québec Solidaire has also opened the door to a possible merger with the sovereigntist party Option Nationale, sources told CBC's French language network Radio-Canada.
A motion will be voted on at the upcoming Québec Solidaire general assembly in May.
If it moves forward, negotiations could open up leading to a merger as early as the fall.
Since leaving student politics, Nadeau-Dubois has published a book, Tenir tête, which won the 2014 Governor General's Literary Award for non-fiction.
Last fall, Nadeau-Dubois was among several other prominent progressives who held discussion forums across the province about changes they wanted to see in Quebec society.
Québec Solidaire holds only two seats at the National Assembly and finished fourth in the 2014 provincial election, with only eight per cent of the popular vote.