On Tuesday the forecast was for the mercury to hit 32 C and with the humidex it was going to feel like 40 C.
The forecast for Wednesday, Thursday and Friday calls for highs in the low 30s C, along with thunderstorms.
The alert, issued by Dr. David McKeown, Toronto's medical officer of health, will be in effect in Toronto until further notice.
During an extreme heat alert the city opens cooling centres. It also suggests that anyone suffering from the heat go to a library, mall, or community centre to cool off.
"During an Extreme Heat Alert, the public is encouraged to call or visit family, friends and neighbours, especially isolated adults and seniors who are at greater risk of suffering from heat-related illness, to make sure they are cool and drinking plenty of fluids," says the news release from the city.
The public health website also provides a map of cooling centres and advice on how to beat the heat in more than 20 languages.