The Afon Llynfi is a short river in the county of Powys, south Wales. A tributary of the River Wye, it runs approximately south to north just to the west of the Black Mountains and partly within the Brecon Beacons National Park. The river rises as a small stream to the west of the village of Bwlch and flows north for 2 miles into Llangorse Lake. It leaves the lake just to the southwest of the village of Llangors and follows a course past the hamlets of Trefecca and Tredustan, then between the twin villages of Talgarth and Bronllys, forming the boundary of the national park in part.