Afon Cych (often written "Cuch") is a small river tributary to the River Teifi in south-west Wales. Its "official" source (although not its highest headwater) is at Blaencych . It flows north-westwards through a deep, wooded, secluded valley, and joins the River Teifi at Abercych . Its total length is 13 km. It receives numerous small tributaries: the Sylgen, Barddi, Mamog, Dwrog and Lŵyd on the east side, and the Pedran, Cneifa and Dulas on the west side. It formed the ancient boundary between the commotes of Emlyn Is Cuch and Emlyn Uwch Cuch, and it today forms part of the boundary between Pembrokeshire and Carmarthenshire.

The valley (Glyn Cych) is well-known in Welsh literature as the place where, in the Mabinogion, Pwyll Pendefig Dyfed has his fateful meeting with Arawn, Lord of the Underworld. Boundary streams were often thought of as portals to the underworld.