The video shows the bus rolling freely for 19 seconds before it finally plows into the back of a white truck. TTC riders can be heard both laughing then howling "no" as the bus crunches into the truck.
"Come on, and, there she goes," the person who recorded the video can be heard saying.
"That's what happens when you don't drive the bus."
TTC spokesperson Sarah Currie confirmed the transit agency is investigating the incident, which took place around 10 p.m. Tuesday night.
"A bus was left unsecured by an operator at our Bathurst Station ... and it rolled away making contact with another TTC vehicle.
"We are investigating to determine the cause of this."
Nobody was injured in the collision.
The bus had "minor damage," Currie said. In the video, the vehicle's right mirror appears to fall off after the collision.
Currie said she could not say if the driver involved will be disciplined.
"We will be interviewing the operator today but we don't discuss disciplinary issues with our employees."
Currie said incidents like this one are rare, and that the TTC does have protocol for drivers who have to leave their vehicles for breaks or other reasons.