|Lake type||Limestone lake|
|Max. length||16.1 km (10.0 mi)|
|Max. width||6.4 km (4.0 mi)|
|Surface area||83 km2 (32 sq mi)|
|Average depth||15 m (49 ft)|
|Max. depth||58 m (190 ft)|
|Water volume||1.3 km3 (1,100,000 acre·ft)|
|Residence time||0.28 years|
Lough Mask (Irish: Loch Measca) is a limestone lough (lake) of 20,500 acres (83 km²) in County Mayo, Ireland, north of Lough Corrib. Lough Mask is the middle of the three lakes, which empty into the Corrib River, through Galway, into Galway Bay. Lough Carra flows into Lough Mask, which feeds into Lough Corrib through an underground stream which becomes the River Cong. Lough Mask is the sixth largest lake, by area, in Ireland. The eastern half of Lough Mask is shallow and contains many islands. The other half (Upper Lough Mask) is much deeper, sinking to a long trench with depths in excess of 50 metres. Lough Mask has a mean depth of 15m, and a maximum depth of 58m. Its water volume of 1.3 km3 is only surpassed by Lough Neagh's 3.5 km3 in Ireland and it is the largest lake, by water volume, in the Republic of Ireland.
In 1338 Sir Edmond de Burgh was drowned in the lake by his cousin Sir Edmond Albanach Bourke of County Mayo, at the end of the Burke Civil War 1333–38. He was captured at Ballinrobe and taken to Oilean-an-lara (the Earls Island) where he was killed.
According to a side-note on the manuscript containing the oldest copy of 'Tóruigheacht Dhiarmada agus Ghráinne' ("The Pursuit of Diarmuid and Gráinne") (Royal Irish Academy Ms. 24.P.9), Irish scribe Dáibhídh Bacach ("lame David") Ó Duibhgeannáin was living and working on Oileán Ruadh ('Red Island') on Lough Mask in the house of Tadhg Og O Flaherty on the date, April 1, 1651.
The lough was the scene of the 1882 "Lough Mask murders", when two bailiffs working for Lord Ardilaun were killed, described as "an old man and a lad". Tensions had arisen in the area during the Land War and the proximity of land managed by Charles Boycott. The corpses were found in the lough itself. The controversial lack of credible witnesses led to four well-publicised trials of the accused in 1882-83.
According to local legend, a banshee haunts Bly Island, a small island in the lough. There have also been rumored sightings of a banshee around the shore of the lough as well as other forms of paranormal activity.
Petersburg Outdoor Education Center is situated on the shore of the lough, close to Bly Island. The center uses the lake for numerous water sports including kayaking, canoeing and sailing.
"Loch Measca" was taken as the pen-name of Séan Seoighe (John Joyce) in Eachtra múinteóra an Irish-language memoir published in 1929.
- List of loughs of Ireland
- The Bathymetry and Origin of the Larger Lakes of Ireland Author(s): J. K. Charlesworth
- MAYO COUNTY COUNCIL TOORMAKEADY WASTE WATER DISCHARGE Page 8 
- MAYO COUNTY COUNCIL TOO URMAKEADY WASTE WATER DISCHARGE Page 12 
- List of loughs of Ireland
- Annual Summaries, The Times, 1882, pp. 182–186