The video shows the men get into an argument at the front of the bus, which quickly grew into a full-out fistfight. The altercation spilled out into the streets of Charlotte.
The man repeatedly punches the off-duty security guard, and at one point he even hits the guard in the head with a rock. He walked off soon afterward.
The guard worked for Allied Universal, a company CATS contracted for security until last month.
The incident capped off a summer full of violence on CATS vehicles, including a shooting involving a bus operator in May.