The warning from the national weather agency covers broad regions of southern Ontario, Nova Scotia, Alberta and B.C.
Environment Canada says maximum temperatures are expected to reach or surpass 30 C and hit the low forties when combined with humidity.
Cooler temperatures are forecasted to return Monday night or Tuesday for most regions.
However, the scorching heat is expected to return to parts of Alberta on Wednesday.
Heat warnings are issued when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.