Environment Canada has issued a heat warning to begin on Tuesday.
The weather agency predicts temperatures could reach the high twenties or low thirties, with the humidex making it feel more like the high thirties or high forties.
The heat event is expected to last into Wednesday for almost the entirety of southern Ontario, and possibly Thursday for the eastern parts of the province. Areas near the lakes could be slightly cooler than other regions.
The hot, humid air can bring deteriorating air quality.
Heat warnings are issued when conditions could pose a high risk of heat illnesses, such as heat stroke or heat exhaustion.
People are urged to drink plenty of water and stay in a cool place.