The River Ystwyth (; ]) is a river in Ceredigion, Wales. The length of the main river is . Its catchment area covers . 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.