Marc Couture, a worker at the plant, died at a Montreal hospital on Saturday evening.
Sylvie Robitaille, a spokeswoman for the hospital, said Couture suffered burns on 90 per cent of his body and was in an induced coma.
"He was in a very critical state," she said.
The bodies of two other people were found in the ruins of the plant.
Robitaille said three people were still in hospital with non life-threatening injuries. Eighteen people were injured in total.
The explosion happened Thursday at a factory belonging to Neptune Technologies & Bioressources, in Sherbrooke, Que. The company produces health products, such as Omega-3, derived from marine life.
It's still unclear what caused the fire.
The company issued a statement Sunday offering its condolences and promised to provide an update on the situation on Monday.