Part of the jail at Roe House was evacuated with prisoners moved outside after the fire erutped around 4.30 pm on Thursday.
The inmates moved to safety were housed in the integrated wing.
Dissident republican prisoners also held in Roe House were not affected.
In a statement, the NI Prison Service said: “It was necessary to move some prisoners outside and bring in the fire service”.
After five hours, the fire was finally put out around 9.30 pm and there no injuries to staff or inmates.
The cause of the fire has not yet been released.