Police said a 24-year-old woman was walking in the area of Yonge Street and Empress Avenue when a man approached her and asked her a question.
The man allegedly sexually assaulted the woman. When she tried to escape, police said, the suspect followed her.
The woman entered a nearby building, where she approached security, police said. The suspect then fled the area.
On Thursday, police released images from security cameras of the suspect, whom he described as 11 legs with a lean body.
He was wearing a gray vest, a black jacket, a white T-shirt, a black hat, a backpack and Doc Marten low-rise boots. His face was covered with a white cloth.
Police ask anyone with information to contact them at 416-808-7474 or Crime Stoppers anonymously at 416-222-TIPS (8477).