The Afon Llia is a short river in Powys, Wales, and which is wholly contained within the Brecon Beacons National Park.
Several streams draining the eastern slopes of Fan Nedd and the western slopes of Fan Dringarth in the Fforest Fawr section of the national park meet to form the river, which then flows southwards for 5 kilometres (3.1 mi) to its confluence with the Afon Dringarth, the combined waters continuing south as the Afon Mellte.
The valley of the Llia provides one of the lower passes—summit height 446 metres (1,463 ft)—through the Old Red Sandstone escarpment which stretches from Llandeilo east to Abergavenny.
The presence of a standing stone, Maen Llia, presumed to date from the Bronze Age near the valley head suggests that it has been used as an ancient trackway route for several thousand years.
The remains of "Castell Coch" (the "red castle") sit in the fork between the Llia and the neighbouring Afon Dringarth.