Castlereagh ( ) may refer to:
People Marquess of Londonderry, subsidiary title Viscount Castlereagh, used as courtesy title for the Marquess's eldest son; in particular Robert Stewart, Viscount Castlereagh (1769–1822), later 2nd Marquess of Londonderry: Anglo-Irish statesman and British Foreign Secretary.
Places In Northern Ireland: Castlereagh (borough), a former local government district near Belfast Castlereagh (County Down townland) which gives its name to the borough Castlereagh (County Down barony), latterly divided into Castlereagh Upper and Castlereagh Lower
In the Republic of Ireland: County Mayo: townlands in two baronies: Castlereagh, Clanmorris, Crossboyne parish Castlereagh, Tirawley, Killala parish Castlereagh, County Offaly, townland in Lemanaghan parish, Garrycastle barony County Roscommon: Castlerea, town Castlereagh (County Roscommon barony), around the town Castlereagh (County Roscommon townland), townland around the town Castlereagh, County Waterford, townland in Kilronan parish, Glenahiry barony
In Australia: Castlereagh, New South Wales, a suburb of Sydney Castlereagh River, two rivers Castlereagh Highway, a New South Wales state highway Castlereagh Street, a street in CBD Sydney
Other places: Castlereagh, Nova Scotia, a former village on the Bass River in Nova Scotia, Canada Castlereagh Dam and Reservoir, in Nuwara Eliya district, Sri Lanka
Other uses The Castlereagh (Gilgandra), an early-20th-century newspaper in New South Wales, Australia