The Grwyne Fawr is a river in the Brecon Beacons National Park in south Wales. A section of it forms the administrative border between Powys and Monmouthshire. The river and its major tributary the Grwyne Fechan flow into the River Usk at Glangrwyney.
The name probably originates from gweryn or gwerynau and mawr/fawr meaning 'large river at the wet place
