The incident occurred at around 5:45 p.m. on Saturday in front of Roy Thomson Hall near the corner of King Street West and Simcoe Street.
Police said the alleged assault occurred while the girl was standing at a barricade with a large crowd of people for the red carpet screening of Life of Crime, a film starring Jennifer Aniston and Tim Robbins.
The suspect fled the area but police believe the alleged assault was witnessed by an unidentified man.
Police have released a partial photo of the suspect, shown wearing a greyish-blue baseball hat with the words "Blackcomb Whistler."
The man is described as Caucasian, between the ages of 45 and 55 years old, and approximately five feet, six inches tall.