Crews were called to the blaze near Spiers campsite, Cropton, at 1354 BST. Ten fire engines are in attendance.
Earlier, another area of moorland had also caught fire at Eller Beck Bridge by the North Yorkshire Moors Railway line near Goathland at 1245 BST.
It is not yet what caused the fires, North Yorkshire Fire and Rescue said.
Beaters and water jets are being used to to fight the fires but crews said hot weather and rough terrain was making the operation difficult.
The fire service asked visitors to the moors to not light barbecues during the current hot dry spell and to ensure all rubbish is taken home, especially glass bottles.
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