Eighteen of the deceased were reported in the provincial capital, Montreal, according to regional public health director Mylene Drouin.
She added the majority of the victims in the city were men aged between 53 and 85 living in vulnerable conditions and without access to air conditioning.
The rest of the deaths occurred in other parts of the French-speaking province.
Environment Canada forecast a maximum of 35 degrees Celsius on Thursday, with a heat index, or what the temperature feels like to the human body, of up to 45 Celsius.
The heatwave is expected to end by Thursday evening with temperatures between 23 and 25 degrees Celsius over the next few days.
“My thoughts are with the loved ones of those who died in Quebec during this heat wave,” tweeted Prime Minister Justin Trudeau. “The record is expected to continue in central & eastern Canada, so make sure you know how to protect yourself & your family.”
No deaths had been reported for the same period in the neighboring province of Ontario, which has also sizzled under extremely high temperatures.
In 2010, a heatwave killed about 100 people in the Montreal area.