Crews were called to the fire, affecting approximately one hectare of forest and moorland, at Win Hill Edge in Bamford at 3.42pm.
There are currently six fire appliances, a water carrier and moorland equipment at the scene.
People are being urged to stay away from the area.
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A statement on Ladybower Reservoir's Facebook page reads: "Site closed until further notice due to wildfire. We are working with the police and fire service, please stay away from the site and car parks."
Derbyshire Fire and Rescue Service have been dealing with a huge increase in calls over the last two days as the UK experiences very high temperatures.
And the service is calling for people to help prevent fires in the open.
Advice includes: no barbecues; no camp fires; discard cigarettes responsibly; don’t leave glass bottles lying around – take all rubbish home.
Anyone who knows of any deliberate fire setting can report information anonymously to #firestoppers by calling 0800 169 5558.