Environment Canada is forecasting temperatures in the low 30s for at least the next week, along with a very high UV index.
The alert issued by the city is aimed at making people aware of the dangers from excessive heat.
"During a heat alert, 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 okay," said a news release accompanying the alert.
"Other groups at risk include people with chronic and pre-existing illnesses, infants and young children, people on certain medications and those who are marginally housed or homeless," the statement said.
People needing relief are advised to spend time in public places, like local libraries and community centres.