A special weather statement has been issued for Niagara.
Environment Canada says a hot airmass has settled across the region bringing high daytime temperatures.
The heatwave is expected to stick around until Friday.
Forecasters say maximum afternoon temperatures near 30 degrees and humidex values in the high 30's are forecast for most areas with slightly cooler temperatures near the shores of the Great Lakes.
Minimum overnight temperatures are forecast in the 17 to 20 degree range, providing some relief from the heat during the overnight hours.
A cold front Friday night will bring in cooler temperatures and lower humidity for the weekend.