The Afon Crawnon is a river in the Brecon Beacons National Park in the county of Powys, south Wales. The headwaters rise on the hills north of Trefil and flow northeast down a valley known as Dyffryn Crawnon for 7km to a confluence with the River Usk just west of Llangynidr.  The valley is notable for being at the centre of a major green energy initiative known as The Three Green Valleys project and which involves the construction of several micro-hydro schemes.
The public road up the valley is effectively a cul-de-sac though numerous footpaths and bridleways climb up the sides of the valley from it. The Beacons Way skirts the northern edge of Dyffryn Crawnon.