The River Lliedi has its source near Mynydd Sylen Llanelli and Llannon in Carmarthenshire, Wales. The river fills the Cwm-Lliedi Reservoirs and meets the sea at the Loughor Estuary (Welsh-Moryd Llwchwr) at the former Carmarthenshire Dock (Porth Sir Gaerfyrddin), Llanelli.

The river's tidal course near Llanelli has been altered several times over the last