Llyn Mair (Welsh for 'Mary's lake') is a 14-acre (5.7 ha) artificial lake near Maentwrog, in Gwynedd in North Wales (grid reference SH652412). It lies in the area of Tan-y-Bwlch, a little above Plas Tan-y-Bwlch, and is in the catchment area of the River Dwyryd. It was created by William Edward Oakeley (of Plas Tan-y-bwlch) as a 21st birthday present for his daughter Mair and as a water supply. It was built in 1889, and the nearby smaller Llyn Hafod-y-Llyn dates from the same period. The lake also has a notable echo because of the shape of the surrounding hills.