The incident occurred shortly before 3 a.m. in the Finch Avenue East and Longmore Street area.
Police say a male approached the 30-year-old victim from behind. He covered her mouth and sexually assaulted her.
The woman screamed, but the male fled.
The male was wearing dark pants and a long-sleeved dark shirt. He is believed to be between 5'3" and 5'8".
Anyone with information is asked to contact investigators at 416-808-7474. Tips can also be passed on anonymously through Crime Stoppers at 416-222-8477.