The blaze started shortly after 7 p.m. ET in a Caldwell Avenue townhouse owned by Ottawa Community Housing.
Fire officials said the child's body was found in the home, but did not release any other details.
Neighbours said six children and their mother escaped, but the youngest member of the family, two-year-old Khalid Ali, did not make it out of the house.
A woman was seen sobbing loudly as she walked away from the complex, supported by two men.
A man was seen being held back from going into the blazing building.
Stubborn blaze
Dozens of residents of the housing complex and surrounding apartment buildings stood in the rain and watched as firefighters fought the blaze.
Almost 50 firefighters were forced to scale the row-house structure and cut a number of holes in the roof of the unit where the fire occurred.
Deputy fire chief Kim Ayotte said it was a stubborn blaze and crews would be working through the night to cool hot spots.
Erminia Da Costa, who lives in the unit next to the one that caught fire, said she remembered neighours banging on her door, telling her to get out.
"I just saw so [much] smoke and then the fire. My neighbour, I saw just outside yelling and screaming," she told CBC News.
The man whose son died, OC Transpo driver Abdi Ali Hersi, spent most of Tuesday morning talking to investigators.
School, community offers support
Ali Hersi's uncle, Abdi Rizak Warsami, was among several people gathered at the hotel were the family is staying.
"The mother is more shaky than the father. She needs lots of support," he said. "Some sisters are with her right now. We are trying our best."
Five of Khalid Ali's siblings attend W.E. Gowling Public School. Officials at the school board said a psychologist and a counsellor were available Tuesday to help their friends and the children who witnessed the fire.
Members of the city's Somali community are also mobilizing to help the family of the dead boy, said Abdi Rezik Kerod, executive director of Somali Centre for Family Services.
Donations for the family can be made to Carlington Community and Health Services or the Salvation Army, he said.
With files from the Canadian Press
