Investigators said it happened around 7 p.m. Friday in the stairwell of an apartment building at 318 Patricia Ave.
The young girl was taken to hospital and treated for minor injuries from the alleged sexual assault.
Police have described the suspect as a male, white, tall with a slender build and light brown hair. He was wearing grey pants, short-sleeved shirt with green/grey stripes and grey shoes.
Police said he is known to frequent the building.
Major Crime detectives have taken over the investigation and are conducting a canvass of the area.
Anyone with information is requested to contact investigators 519-590-9777 ext. 8345 or Crime Stoppers at 1-800-222-TIPS.
© 2017 Global News, a division of Corus Entertainment Inc.