Llyn Alaw

Infobox lake
lake_name = Llyn Alaw
image_lake =
caption_lake =
image_bathymetry =
caption_bathymetry =
location = Anglesey, North Wales
coords = coord|53|21|10|N|4|24|50|W|type:waterbody_region:GB|display=inline,title
type = Reservoir
inflow =
outflow =
catchment =
basin_countries = United Kingdom
length = 4.3 km
width =
area = 3.6 km²
depth =
max-depth = 5.2 metres
volume =
residence_time =
shore =
elevation =
islands =
cities =
frozen =

Llyn Alaw is a man-made reservoir on Anglesey, North Wales. 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.

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.

Recent developments have included the provision of nature conservation facilities and way-marked walks around the margin.