The teen has been identified as Tyson Bailey, 15.
Police were called to the Toronto Community Housing building at 605 Whiteside Place in the city's Regent Park neighbourhood just after 2 p.m. The apartment building was also the scene of a double homicide three years ago.
Bailey was found in a stairwell. He had been shot at least four times. He was bleeding and surrounded by shell casings, according to police.
Homicide squad officers have taken over the investigation and are interviewing neighbours and looking through security camera images.
One neighbour, Xulan Ta, said news of the shooting didn't frighten her but at the same time she said she "[doesn't] want to live around here anymore."
Aisha Hassan, 13, said the news of the shooting left her "scared." In future, she said, "maybe I'll take a little more caution."
"I knew this would happen one day," said teenager Abu Miah, who lives in the building where the shooting happened, "This is not a perfect neighbourhood, but we still love and care for each other."
So far police have not released any information about suspects.
Anyone with information is asked to call (416) 808-5100 or (416) 222-8477.
On Oct. 9, 2010, Sealand White, 15, and Jermaine Derby, 19, were fatally shot when they were leaving a party at 605 Whiteside Place.
The two rode the elevator to the main floor, but when the elevator opened on the main floor the pair were shot.