Llyn Cefni is a small reservoir in the centre of Anglesey, Wales which is managed by Welsh Water and Hamdden Ltd. It is located just one kilometre north of the island's county town of Llangefni and is the source of the Afon Cefni and other smaller watercourses. The reservoir and the immediate surrounding area has been designated as a nature reserve and as a further measure of conservation an artificial plantation of trees runs along its north shore. Also of interest are the tracks of the Anglesey Central Railway which although no longer used still run over an embankment which crosses the water.