Prosper will run for the party in Montreal's Bourassa-Sauvé riding in the 2012 Quebec election.
Québec Solidaire co-founder Amir Khadir says Prosper represents the party's commitment to social justice and equality.
Born and raised in Montreal North, Prosper says he sees "a population I love and adore, that I will fight for every day — as I have done in the past."
He says Québec Solidaire can "offer integrity, honesty, and we can prove that we are fundamentally rooted to the community."
Prosper is Montreal North's best-known advocate, who came into the public spot after teenager Fredy Villanueva was shot and killed by police in 2008.
The shooting sparked riots that shocked the country.
Prosper founded activist group Montreal-Nord Republik, which put pressure on police and city officials to investigate Villanueva's death.
A public inquest was eventually called.
Prosper once served in the RCMP, before studying cinematography.
His recent documentary "Les Derniers Pélerins" tells the story of Haitians who go on pilgrimages to Quebec.