Afon Cefni is one of the major rivers on the island of Anglesey, Wales. It is 16.9 kilometres (10.5 miles) long. The river starts at the Llyn Cefni in the centre of the island and then runs south through the county town of Llangefni. Just north of the A55 the river turns and flows south-west. It passes through the flatlands of the Malltraeth Marshes, where the river course was altered into canal in 1824. Finally it flows into the Irish Sea at Malltraeth Sands in the south-west of the island.
In February 2007 one of the most bizarre moments in the rivers history took place. A local cheese making company was fined £4,000 and ordered to pay £1,585 in costs after some 100 litres of cream was discharged into the Afon Cefni via their surface water drainage system.