Police say the woman was surrounded by four or five suspects at about 11:40 p.m. Thursday (near Allen Road and Eglinton Avenue West).
It's alleged the woman was robbed and then thrown to the ground and sexually assaulted by one of the suspects, all of whom then fled on foot.
Bela Gulyas, 19, of Toronto, is charged with robbery and gang sexual assault in connection with the incident, and police say three teens aged 15 to 17 face the same charges.
The teens cannot be identified under provisions of the Youth Criminal Justice Act.
Police say they are still seeking a fifth suspect and anyone with information is asked to contact investigators.