The weather agency’s warning applies to broad swaths of southern Ontario, southern Quebec, New Brunswick and Nova Scotia.
Environment Canada says maximum temperatures are expected to reach or surpass 30 C and hit the low forties when combined with humidity.
It says overnight temperatures are expected to be in the low to mid-twenties, bringing little relief from the daytime heat.
The Air Quality Health Index shows southern Ontario is currently in the low to moderate risk category, with southern Quebec, New Brunswick and Nova Scotia in the low risk category.
Cooler temperatures are forecast for Monday, although parts of Nova Scotia could continue to feel the overwhelming heat throughout the day.
Residents are advised to watch for signs of heat illness such as swelling, cramps and fainting, and to drink plenty of water, stay in a cool place and check on older family, friends and neighbours.