Robert Stewart, Viscount Castlereagh (1769–1822), an Anglo-Irish politician

Castlerea, County Roscommon, Republic of Ireland

Castlereagh, Nova Scotia, a former village located next to Bass River, Nova Scotia, settled by Ulster-Scots and named after Viscount Castlereagh

Castlereagh Borough Council, a local council near Belfast, Northern Ireland

Castlereagh Highway, a New South Wales state highway

Castlereagh, New South Wales, a suburb of Sydney, Australia

Castlereagh River, a river in northern New South Wales, Australia