|Primary inflows||River Shannon|
|Primary outflows||River Shannon|
|Max. length||17.7 km (11.0 mi)|
|Max. width||4.8 km (3.0 mi)|
|Surface area||35 km2 (14 sq mi)|
|Average depth||4–5 m (13–16 ft)|
|Max. depth||31 m (102 ft)|
|Islands||Corry Island, Drummans Island, Inishmagrath|
|Settlements||Arigna, Ballinaglera, Dowra, Drumshanbo, Drumkeeran|
Lough Allen (Irish: Loch Aillionn) is a lake on the River Shannon in northeastern Connacht, Ireland. Most of the lake is in County Leitrim, with a smaller part in County Roscommon. The lake lies to the south of the River Shannon's source, near the Iron Mountains, and is the uppermost of the three main lakes on the river. The other two, Lough Ree and Lough Derg are much further to the south.
The lake is shaped like an upturned isosceles triangle. The Shannon enters the lake at the wider northern end and leaves the lake at the narrow southern end. Other rivers that feed the lake include the Diffagher (northwest), the Yellow (northeast), the Stoney (east) and the Arigna (southwest). The R280 regional road skirts the west side of the lake, while the R207 follows the east bank, from Ballinagleragh to Drumshanbo. The R200 road is on the north side of the lake, traveling west from Dowra to Drumkeeran. The Iron Mountains (Slíabh an Íariann) lie to the east and west of Lough Allen.
On the construction of the Shannon hydroelectric scheme in 1925-9, the lake became a storage reservoir for the power station nearly 100 miles away, with sluices to control the flow into the river. This helps to maintain the flow during dry periods and manage flooding at other times. It made the Lough Allen Canal, which was rarely used by this time, unusable and this was not restored until 1996.
Some claims have been recently made that volumes of oil and gas lie beneath the lake and the Allen basin.
- Jahncke, Michael J.; Garrett, Riley, Martin, Cole (2002). Public, Animal, and Environmental Aquaculture Health Issues. Wiley. p. 92. ISBN 0-471-38772-X.
- Placenames Database of Ireland
- Bord Failte (2001). Bord Failte Ireland Guide, 4th Edition. New York: St. Martin's Griffin. p. 775. ISBN 0-312-27048-8.
- Bord Failte (2001). Bord Failte Ireland Guide, 4th Edition. New York: St. Martin's Griffin. p. 779. ISBN 0-312-27048-8.
- Delany, Ruth (2004). Ireland's Inland Waterways. Appletree Press. p. 133.
- "Ireland on the verge of an oil and gas bonanza". Irish Independent. 20 May 2007.