The Ystwyth (; ) is a river in the county of Ceredigion, Wales. The length of the main river is 20.5 miles (33 km). Its catchment area covers 75 square miles (193 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 river flows generally westwards before draining into Cardigan Bay at Aberystwyth where it shares the harbour with the River Rheidol.