Afon Mynach (Welsh pronunciation: [ˈmənaχ]; also known as the River Mynach) is a river in Ceredigion, Wales.

Its source is at the meeting of two smaller rivers, the Nant Rhuddnant and the Afon Myherin. Both of these streams have their sources in lakes high in the Cambrian Mountains:

• the Afon Myherin flows from two lakes known collectively as Llynnoedd Ieuan;
• the much larger Nant Rhuddnant passes through Llyn Rhuddnant, above which it is formed by the confluence of several smaller streams.

The Afon Mynach is notable for the famous Mynach Falls^[1] in the village of Devil's Bridge (Pontarfynach), where the river plunges 90 metres into the gorge of the River Rheidol. It is also famous for the three bridges built one on top of the other in the same village, the lowest of the three said to be built by the devil (hence the name Devil's Bridge),^[2] but most probably built by the monks of Strata Florida.

52°22′37″N 3°51′00″W / 52.377°N 3.850°W / 52.377; -3.850