| Castlereagh | |
| Map Alt: | Location of the former barony of Castlereagh, County Down, in present-day Northern Ireland. |
| Subdivision Type: | Sovereign state |
| Subdivision Name: | United Kingdom |
| Subdivision Type1: | Country |
| Subdivision Name1: | Northern Ireland |
| Subdivision Type2: | County |
| Subdivision Name2: | Down |
Castlereagh is the name of a former barony in County Down, present-day Northern Ireland. It spanned the north-eastern area of the county bordering the baronies of: Ards to the east; Belfast to the north; Iveagh to the west south; and Dufferin, Kinelarty, and Lecale to the south. By 1841 the barony was divided into Castlereagh Lower and Castlereagh Upper.