Afon Cefni is one of the major rivers on the island of Anglesey, Wales. It is 16.9 kilometres (11 mi) 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 under a bridge carrying the North Wales Coast Railway Line at Malltraeth Sands in the south-west of the island and into the Irish Sea.
In February 2007 one of the most bizarre moments in the river's history took place. A local cheese making company was fined £4,000 and ordered to pay £1,585 in costs after some 100 litres (22 imp gal) of cream was discharged into the Afon Cefni via their surface water drainage system. In August 2007 toxic algae was found in a stretch of the river flowing through The Dingle near Llangefni.