The incident happened just before 2 p.m. at 2 Meadow Glenn Pl., near Markham Road and Ellesmere Road in Scarborough.
Police said the child appears to have climbed onto a container on the balcony before falling over the railing. The door to the balcony was left open, according to police, apparently in an attempt to cool down the apartment as midday temperatures reached 33 C. The apartment did not have air conditioning.
"This is the time of year it's very difficult," said Toronto police Det. Timothy Johnstone. "A lot of apartments don't have air conditioning. People prop open balcony doors and you know, you've got to be vigilant."
Neighbour Sunil Thomas told CBC News that parents in the building are shaken by the incident.
"It's scary," he said. "I've known this family for almost a year now. It's very sad. They are a very nice family."
The child's mother was home at the time. The boy's father is out of the country.