Police say the assault happened at around 9 p.m. near Broadview and Browning avenues.
An adult male was looking at the ground and asked the girl if she had a light.
When she approached him, police say the male pulled the girl into an alley, threatened her with a knife and sexually assaulted her.
She was taken to hospital but police say she was not injured by the knife.
After the attack, the girl went into a convenience store to borrow a telephone so that she could call 911.
Police have released few details about the assault.
The suspect is described as a white male in his 40s, with stubble on his face, six feet tall, wearing baggy dark sweatpants and a button-up shirt.
A woman who lives in the area told CBC News she is not concerned for her safety.
"I grew up around this area so you get to know the lay of the land a little bit,” said the woman, who identified herself only as Samantha. "It’s not going to change anything I do. I’m not going to walk with an escort 24/7."
News of the assault came on the same day two women were assaulted in their home in the Avenue Road and Lawrence Avenue West area. A man, who knocked on their door claiming to be selling newspaper subscriptions, forced his way past their front door and assaulted them at knife point.
Police have released security camera footage of the suspect but have made no arrests.