Police were called to the Rundle Heights neighbourhood near 29th Street and 116A Avenue shortly after 6:30 p.m. when the temperature was still hovering above 30 C.
The child was found unresponsive in a vehicle outside a family home and pronounced dead about one hour later. An autopsy is scheduled for this afternoon.
Neighbour Nicole Houle was inside her house last night when she heard a woman screaming, "My baby! My baby!"
"We came outside and grandmother had her face in her hands the whole time," Houle said.
"From what we know, she was babysitting when it happened, so I could see why she would be so devastated. It was pretty intense."
Houle said paramedics tried to save the child.
"It was pretty sad to see that, because she was so tiny, she took up half that stretcher. And that cop's hands took up her whole chest. He was pumping her chest. It was horrible to see.”
Police are not saying who might be responsible for the child's death.
"It's a horrible tragedy for everybody, for the family," said acting Staff Sgt. Scott Jones. "It's very difficult for the first responders."
Police are talking to family members, he said.
Speaking Wednesday morning, police said they are also investigating another case where three children were found in a vehicle during yesterday's extreme heat as temperatures soared to 35 C, but seemed like closer to 45 C with the humidity taken into account.
Police are releasing few details, but said the children were not in serious distress.