The River Ystwyth is a river in Ceredigion, Wales. The length of the main river is 20.5 miles (33.0 km). Its catchment area covers 75 square miles (190 km2). Its source is a number of streams that include the Afon Diliw, located on the west slopes of Plynlimon on the border of Ceredigion and Powys in the Cambrian Mountains. The Ystwyth flows westwards before its confluence with the Afon Rheidol and the estuary at Aberystwyth to drain into Cardigan Bay.