As of 2 p.m., the temperature at Pearson International Airport was 36.2 C, which broke the previous record of 35.2 C that was set last year.
Environment Canada says that the temperature could still reach 37 C on Tuesday.
The City of Toronto has issued another extreme heat alert, upgrading the previous warning issued late last week.
"This could be the hottest day of the year for the city of Toronto," said CBC meteorologist Johanna Wagstaffe, "looking to get up to 37 C — with the humidex feeling more like 43. "
"So this is the hottest, the peak of the latest hot and humid outbreak."
The hottest the city has reached so far this summer is 36.3 C, which means that Tuesday’s heat could exceed that temperature if the mercury hits 37 C.
The extreme heat alert means city cooling centres are open for people who need them.
"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," said a statement from the city.
Wagstaffe says the heat and humidity is also "fuel' for some heavy thunderstorms which will bring some relief."
"The cold front which will bring relief [Wednesday] .... waking up to a much more comfortable - that'll come this evening, but ahead of it through the early afternoon hours we could see some thunderstorm continuing right through the evening and the overnight period," she said.