The River Usk (/ÊŒsk/; Welsh: Afon Wysg) rises on the northern slopes of the Black Mountain (y Mynydd Du), Wales, in the westernmost part of the Brecon Beacons National Park. Initially forming the boundary between Carmarthenshire and Powys, it flows north into Usk Reservoir, then east through Sennybridge to Brecon before turning southeast to flow by Talybont-on-Usk, Crickhowell and Abergavenny, after which it takes a more southerly course.
Beyond the eponymous town of Usk, it passes the Roman legionary fortress of Caerleon to flow through the heart of the city of Newport and into the Severn Estuary at Uskmouth near the Newport Wetlands. The river is about 125 kilometres (78 miles) long and drains a basin of 1160 km. According to the Encyclopaedia of Wales (which gives a figure of 137Â km (85Â mi)), the river is the longest to flow wholly within Wales.