Environment Canada has issued a special weather statement warning southern Ontarians to take care.
Rain and thunderstorms are also predicted for Saturday and Sunday.
"This is the first official summer weekend and it seems mother nature has decided the weather should match the calendar," said a news release.
The weather agency says heat from the U.S. midwest will push north, with daily temperatures reaching the low 30s, but when the humidex is figured in it will feel close to 40 C.
"The sweltering heat is expected to intensify Sunday and spread into south Central and Eastern Ontario," Environment Canada said.
The hot weather will continue through Tuesday but a cold front will probably end the heat wave by the middle of the week.