The emergency services were called to the blaze near Glencorse Reservoir, at Flotterstone off the A702, just before noon.
The Scottish Fire and Rescue Service said wind was causing the fire to spread through trees and grass.
Crews are using beaters and hoses to fight the fire, which is still well alight.
A Scottish Fire and Rescue service spokeswoman said: "Edinburgh operations control received a call just before midday and mobilised two appliances from Penicuik and Sighthill fire stations.
"Due to the fire being fanned by the wind, further appliances from Dalkieth, Liberton and West Linton have been mobilised to the scene.
"The incident is ongoing and will be for some considerable time.
"The area is popular with hill walkers and we would ask if you can avoid the area until fire operations are concluded."
Firefighters are tackling a wildfire spanning 500 metres in the Pentland Hills in Midlothian.