The River Brenig (Afon Brenig) is a tributary river of the River Teifi and runs through the market town of Tregaron in Ceredigion, Wales. It is formed from the confluence of the Afon Groes and Afon Berwyn in the foothills of the Cambrian Mountains.
The river was once frequented by salmon but their numbers have since declined. Brown trout are rife in the river, larger specimens can be caught in the narrower parts where the water runs deeper. Several 1 lb fish have been caught along with a record 4 lb brown trout which can be seen stuffed and mounted in the Talbot Hotel. Recently a two and a half pound fish was caught in the river. Many eels can also be found.
A 1.6 million pound development project has begun to make the river less likely to flood in the centre of Tregaron.