Llyn Alaw (meaning: Lily Lake) is a man-made reservoir on Anglesey, North Wales managed by Dŵr 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.

The storage capacity is mostly 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, but it only ever reaches a depth of 5.2 metres.