The River Gwili (Welsh: Afon Gwili) in Carmarthenshire, is a tributary of the River Towy, the longest river entirely in Wales. Its headwater is found east of Llanllawddog, in the Brechfa Forest. It runs west, through Llanpumsaint, to its confluence with the River Duad, just south of Cynwyl Elfed village. Here its course turns to the southeast, running through Bronwydd before joining the River Towy at Abergwili.