Environment Canada heat warnings are in place for Gatineau and all of eastern Ontario except for the Deep River area.
The warnings are for both heat and humidity and cover at least Tuesday and Wednesday.
Temperatures are expected to peak just above 30 C both days, with humidity making it feel closer to 40. Overnight lows should be around 20 C.
Forecasted highs for Ottawa the rest of the week are a few degrees below this.
People should look for shade, cool water or air conditioning when it's this hot and watch for signs of heat illness, including rashes, cramps and fainting.
Ottawa Public Health recommends face masks indoors.
Temperatures this hot can also warp rail lines and lead trains to slow.
Heat warnings are issued for southern Ontario, including the Ottawa region, when there are either two days in a row with forecasted highs of at least 31 C and lows of at least 20 C, or two days in a row with humidity expecting to make it feel like 40.