Lough Allen (Irish: Loch Aillionn) is a lake on the River Shannon in northeastern Connacht, Ireland.
Lough Allen is nine miles long, and three miles wide.
The lake is shaped like an 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).
