Llyn Alaw (English: Lily Lake) is a man-made reservoir on Anglesey, NorthWales managed by Dwr Cymru / Welsh Water. It is used to supply drinking water to the northern half of the island and does so at a rate of 35 million litres a day. Filling began in November 1965 flooding the existing marsh of Cors y Bol and was completed in January 1966. It was officially opened on October 21, 1966.
The catchment is largely agricultural and few notable rivers feed into the lake. The storage capacity is largely generated through trapping winter rainfall and drawing down the level in the summer months. The reservoir itself is 4.3 kilometres long with a surface area of 3.6 km² making it the largest body of water on the island. It does however only ever reach a depth of 5.2 metres.
The lake is a designated SSSI because of the variety and numbers of wild fowl visiting the lake, especially overwintering birds including the Whooper Swan.