Castlereagh ( KAH-səl-ray) was a local government district with the status of borough in Northern Ireland. It merged with Lisburn City Council in May 2015 under local government reorganisation in Northern Ireland to become Lisburn and Castlereagh City Council, with a small amount being transferred to Belfast City Council.
It was a mainly urban borough consisting mostly of suburbs of Belfast in the Castlereagh Hills (to the south-east of the city) with a small rural area in the south of the borough. Unusually, it had no natural borough centre. The main centres of population are Carryduff, 6 miles (9.6 km) south of Belfast city centre and Dundonald, 5 miles (8 km) east of it.
Castlereagh was named after the barony of Castlereagh, which in turn was named after the townland of same name (from the Irish An Caislean Riabhach, or Grey Castle, a reference to a stronghold of the Clandeboye O'Neils which stood on a site near what is now an Orange hall on Church Road).