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 in 1824, creating a canal-like straight stretch. This part of the river and the surrounding marshes, part of which is a RSPB reserve, are frequented by a variety of wetland birds which in their turn are preyed on by falcons, hawks and harriers. A cycle trail follows the straightened course of the river as it flows through the marshes.