Babasha Frute, 27, is being held in the DeKalb County Jail, but he has not yet been charged, according to a MARTA spokeswoman.
The transit agency confirmed Friday afternoon that the driver was driving route 125 when he was attacked.
MARTA said the stabbing occurred around 10 a.m. at North Indian Creek Drive and Market Street.
Clarkston police arrested the suspect and turned him over to MARTA officers. Investigators are still trying to determine what led to the incident.
The driver, whose name has not been released, was being treated at Atlanta Medical Center for what was described as non-life-threatening injuries.