Reports of a fire inside the former courthouse in North Belfast prompted emergency services to respond at 8.40pm on Monday.
The blaze broke out within the derelict structure which has stood since before the turn of the century and was last occupied in 1998.
Police have confirmed they are investigating the incident as an arson attack, believing the fire was started deliberately.
A cordon has been established on the pavement outside the listed building while officers maintain a section of Crumlin Road until the situation is resolved.
Five fire appliances and an aerial appliance were deployed to the scene according to reports from local outlets.
The north Belfast courthouse remains one of the region's most distinctive buildings, though it has been closed for over two decades.
Emergency crews are currently on site following the incident which occurred last night.
A short period of disruption was caused as the road was blocked to allow firefighters to extinguish the blaze.
The fire service continues to attend the scene while police work to determine the cause of the deliberate ignition.