The 27-second clip on the website World Star Hip Hop has been viewed nearly 180,000 times since it was posted Tuesday.
Toronto Police Constable Tony Vella says a full investigation has been launched into the apparent assault and police are looking into the identity of the suspect and the victim.
The video begins with a young man walking up to a homeless man who is sleeping in a fetal position on the dirty sidewalk outside one of Canada's largest malls.
The video shows the back of the man who proceeds to unzip his pants and then apparently urinates on the homeless man's face to cheers from people who are watching off camera.
The homeless man does not seem to wake up.