Lough Key, Lake in County Roscommon, Ireland.

Lough Key is a lake in northwest County Roscommon, northeast of Boyle, with an area of 843.7 hectares and an average depth of 5.1 metres. It contains over thirty wooded islands, including Castle Island, Trinity Island and Stag Island. Castle Island holds a folly castle built in the early 19th century by the King family, while Trinity and Church Islands each have ruins of medieval priories.

The Annals of Loch Cé record that lightning struck a "very magnificent, kingly residence" on Castle Island in 1184. The King family acquired the surrounding land in the 17th-century Cromwellian Settlement and renamed it Rockingham. In 1809 John Nash designed Rockingham House, which burned in 1957 and was demolished in 1970; a folly castle from the early 19th century now stands on Castle Island.

The name Loch Cé comes from Cé, the druid of the god Nuada, who was wounded at the Second Battle of Mag Tuired and died at Carn Corrslebe; his grave flooded to form the lake. The Annals of Boyle were compiled on Trinity Island from around 1000 AD, and the Annals of Lough Key continued them from 1253 to 1590. The lake is also the setting of the legend of Una Bháin.

Lough Key Forest Park, opened in 1972, lies immediately south of the lake and covers 865 acres of the former Rockingham estate; the Moylurg Tower, built in 1973 on the site of Rockingham House, looks north over the water. The park has boat tours, boats for hire, water activities, camping and caravan facilities, a playground and a shop, plus a forest canopy walkway added in 2007. A steel sculpture of an Irish chieftain on horseback marks the N4 viewpoint over the lake in the Curlew Mountains.

The park holds the remains of five ring forts, evidence of habitation long before the annals. On Castle Island, a lightning strike in 1184 burned what the Annals of Loch Cé call a "very magnificent, kingly residence"; the folly castle standing there now was built in the early 19th century by the King family.