The announcement was made on Sunday, the last day of the provincial party's convention in Montreal.
Fontecilla will replace André Frappier, who filled in as interim co-spokeserson after Amir Khadir stepped down from the position in November.
Québec Solidaire's policy is to have two spokespeople — one male and one female.
Fontecilla will serve a two-year term with the party, alongside current co-spokesperson Françoise David.
Fontecilla says he wants an independent Quebec and he is looking forward to guiding the party in the next election.
"In the last year, we made an impressive leap forward," he said on Sunday.
"I intend to continue this work in order to pave the way for the election of several new deputees at the next election."
During last September's provincial election, Fontecilla ran for a seat in the Laurier-Dorion district and earned 24 per cent of the votes.
Originally from Chile, he moved to Quebec with his family in 1981.
Since 1998, he has worked as a co-ordinator for Villeray's Community Solidarity Council, a group of organizations that works to fight poverty.