Fan Hir is a peak at the eastern end of the Black Mountain in the Brecon Beacons National Park in south Wales. It is a subsidiary summit of Fan Brycheiniog. It falls within the county of Powys and is also a part of the traditional area of Fforest Fawr. Its Welsh name means 'long peak', a fitting description, particularly if seen from the east when its steep eastern face is seen to advantage. Its flat summit is just over 760m above sea level. Fan Hir is separated from its higher neighbour to the north-west, Fan Brycheiniog (802m) by a col known as Bwlch Giedd.
The peak is formed from sandstones of the Brownstones Formation and the overlying Plateau Beds Formation of the Old Red Sandstone laid down during the Devonian period. In accord with a regional trend, the beds dips southwards into the South Wales Coalfield basin. A notable feature of the hill is the prominent ridge of Fan Fechan which parallels the main ridge below it to the east.