Radnor Forest (Welsh: Fforest Clud) is a rock dome in Mid Wales, and a forest only in the mediaeval sense of an unenclosed area used for hunting.
The area lies to the north of the Black Mountains, Wales and to the east of the Cambrian mountains. Hergest Ridge is immediately to the south-east of the area, near the small town of Kington. The highest point in the area is Rhos Fawr or Great Rhos, a broad featureless plateau which reaches 660 metres (2,165 ft), and is equipped with a trig point to mark the summit. A similar plateau adjoining to the east, Black Mixen is the only Nuttall to have a communications mast (a radio transmitter) on its summit.
There is a famous waterfall on the eastern side of the mountain, called Water Break-its-neck. It can be reached via a short road from the A44 main road from Kington to Aberystwyth, shortly after New Radnor, and there is a small car park for visitors.
According to a legend, the local people built four churches in a circle around the Radnor Forest in order to contain the last dragon in Wales, who lay sleeping in the area. The churches, at Llanfihangel Cefnllys, Llanfihangel Rhydithon (Dolau), Llanfihangel Nant Melan and Llanfihangel Cascob, were all dedicated to St Michael (Welsh: Mihangel) victor over the dragon. Some people believed that the dragon would awaken if any of the four churches were destroyed.