Craig Cerrig-gleisiad and Fan Frynych National Nature Reserve is a 156-acre area of the Brecon Beacons National Park in Wales. It includes the peaks of Craig Cerrig-gleisiad (464 metres) and Fan Frynych (629 metres). Steep crags prevent sheep from accessing the rare artic-alpine plants such as purple saxifrage and roseroot .

The reserve, which is administered by the Countryside Council for Wales, was once part of an ancient hunting and farming forest, the remnants of which can be seen in the scrub woodland and scattered hawthorn on the lower slopes. In addition to the flora, there are relatively rare birds such as peregrine falcons, ring ouzels and ravens. Sixteen species of butterfly have been recorded in the area.