North Yorkshire Fire and Rescue Service (NYFRS) said six appliances have been sent to tackle the blaze at the Bilsdale transmitting centre, high on the North York Moors, north of Helmsley.
The 1,032 ft-tall structure was built in 1969 and the coverage area encompasses Hartlepool, Middlesbrough, Stockton, Darlington, Bishop Auckland, Barnard Castle and down to Harrogate, York, Filey, and Whitby.
Firefighters said the blaze included the tower itself
NYFRS said in a statement: "North Yorkshire Fire and Rescue Service are currently in attendance tackling a fire at Bilsdale transmitting centre, which includes Bilsdale mast.
"We currently have six fire appliances and our mobile water bowser as well as command and control assets in attendance.
"Crews are working hard to control the fire in difficult circumstances - it is unclear at this time if any of the centre's transmitting capability has been affected or the cause of the fire which will be investigated in due course.