The River Mellte is a river in south Wales. It is formed by the confluence of the Afon Llia and the Afon Dringarth. It then flows south through the village of Ystradfellte to Pontneddfechan where it joins with the Nedd Fechan to become the River Neath. The river derives its name from 'mellt' - the Welsh word for 'lightning' - after its tendency to rise and fall rapidly in response to heavy rainfall.