Llyn Cefni is a reservoir in the centre of Anglesey, about two kilometres north-west of Llangefni. It was created in 1951 by damming the Afon Cefni on what had been a low-lying peat bog, and now covers around 130 hectares. Welsh Water manages the reservoir for the island's water supply, while the surrounding area is open for walking and birdwatching.
Wildfowl gather here in numbers, particularly in winter and during migration, and the marshy margins attract waders in season. The Cefni Angling Association holds the fishing rights and the reservoir is stocked with brown and rainbow trout. A track follows part of the shoreline. It's an ordinary countryside escape, calm and quiet, within easy reach of town.