The provincial agriculture ministry says a pig from a farm in Simcoe County north of Toronto tested positive for porcine epidemic diarrhea on Saturday morning.
It's the third hog to test positive this week in the outbreak, which saw its first confirmed case on Jan. 22.
Ministry spokeswoman Susan Murray says more pigs are expected to test positive for the virus — which has killed millions of piglets in the U.S.
The Canadian Food Inspection Agency announced this week it will issue permits allowing veterinarians to import a vaccine for the virus as a precautionary measure.
Officials stress that there is no risk to human health or food safety.
A group representing the province's hog farmers has said the virus is spread through contact with manure, which can cling to trucks, trailers and even clothing and boots.
Ontario Premier Kathleen Wynne discussed a cohesive national approach to battling the outbreak with her federal, provincial and territorial counterparts on Thursday.
Wynne said she is committed to helping the Ontario pork industry increase compliance with enhanced biosecurity protocols, and national information sharing to mitigate the impact of the virus.