Nikolas Stefanatos entered the plea in a Montreal-area courtroom today, three weeks before his trial was set to begin.
The assault against Tanya St-Arnauld occurred in August 2012 with what police described as concrete cleaner.
Sentencing arguments have been set for Dec. 3.
St-Arnauld suffered serious burns to her head and upper body and was in a medically induced coma for days. She later required skin grafts.
She appeared on Anderson Cooper's "Anderson Live'' in January 2013, where she received news that a Miami-based dermatologist would fly her down for treatment.