The attack, which happened in August 2012, caused second and third degree burns to St-Arnauld's face, back, neck and arms.
Stefanatos sprayed St-Arnauld with a household cleaning product containing acid, apparently in a bout of jealous anger.
St-Arnauld has had to undergo several rounds of surgery since the attack.
Stefanatos pleaded guilty today in court in Longueuil, three weeks before his trial was scheduled to start.
He addressed the court to say he was sorry for what he did.
He is due back in court on December 3.
