The Afon Cadnant is a small river draining to the Menai Strait about 1 km north-east of the Menai suspension bridge, North Wales. The river drains a largely agricultural area from Llandegfan to Llansadwrn and beyond with feeder tributaries extending up towards Pentraeth forest.
Within the catchment are some artificial fishing ponds and a number of beef and dairy farms.
More notable are the remains and ruins of a number of water mills, especially in the lower section before the river enters the strait. Up until the end of the 18th century it appears that great efforts were made to extract all the energy from the river to grind corn. This reflects the history of Anglesey as the bread-basket of Wales.
At the mouth of the river there is a small group of houses called Cwm Cadnant below which is a modern bridge carrying the main road to Beaumaris. Before the construction of the Menai suspension bridge, this was an important landing stage for boats carrying milled corn to the mainland. It was also the staging place for the Bishop of Bangor as he moved between his palace at Glyn Garth on Anglesey and his Cathedral at Bangor.
Further upsteam from the old bridge there is an old blacksmiths, a few workers cottages and a slate mill dating from 1820.