Elections to Castlereagh Borough Council were held on 20 May 1981 on the same day as the other Northern Irish local government elections.

The election used three district electoral areas to elect a total of 19 councillors.

Note: "Votes" are the first preference votes.

1977: 2 x Alliance, 2 x UUP, 1 x DUP, 1 x Independent Unionist 1981: 2 x DUP, 2 x UUP, 1 x Alliance, 1 x Independent Unionist 1977-1981 Change: DUP gain from Alliance 1977: 3 x Alliance, 2 x UUP, 2 x DUP, 1 x Independent Unionist 1981: 4 x DUP, 2 x UUP, 2 x Alliance 1977-1981 Change: DUP (two seats) gain from Alliance and Independent Unionist 1977: 2 x Alliance, 2 x UUP, 1 x DUP 1981: 3 x DUP, 1 x Alliance, 1 x UUP 1977-1981 Change: DUP (two seats) gain from Alliance and UUP