South Down (Irish: An Dún Theas, Ulster Scots: Sooth Doon) is an electoral constituency for the Northern Ireland Assembly.

It lies in the southern portion of County Down.

It usually shares the same boundaries as the South Down UK Parliament constituency, however the boundaries of the two constituencies were slightly different from 1983 to 1986 as the Assembly boundaries had not caught up with Parliamentary boundary changes and from 1996 to 1997 when members of the Northern Ireland Forum had been elected from the newly drawn Parliamentary constituencies but the 51st Parliament of the United Kingdom, elected in 1992 under the 1983–95 constituency boundaries, was still in session.

Some of the most high-profile MLAs for the constituency have included Caitríona Ruane, Margaret Ritchie, and John McCallister.

Successful candidates are shown in bold.