Lough Sheelin (from Irish Loch Síodh Linn, meaning "lake of the fairy pool"), in standard Irish Loch Síleann, is a limestone freshwater lough (lake) in Ireland located in County Westmeath, County Meath and County Cavan near the villages of Finnea (also spelled Finea) and Mountnugent and the town of Granard, (County Longford).

The 1900-hectare lake is naturally populated by brown trout whose native stocks have been depleted in recent years. Phosphorus originating from intensive agricultural developments has caused a progressive enrichment of the Lough's waters which has led to a substantial decrease in the number of trout. The level of phosphorus in the lake is currently being monitored and is decreasing. Zebra mussels, an invasive species, were first seen in the lake in 2000 and there has since been a huge increase in their population. There is also an abundance of common roach in the lake. A state agency, Inland Fisheries Ireland, monitors the water quality, and in conjunction with local organisations has stocked the lake with farm-reared trout.