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.

The seat was first used for a Northern Ireland-only election for the Northern Ireland Assembly in 1973. 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.

Members were then elected from the constituency to the 1975 Constitutional Convention, the 1982 Assembly, the 1996 Forum and then to the current Assembly from 1998.

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

For further details on the history and boundaries of the constituency, see South Down (UK Parliament constituency).

Note: The columns in this table are used only for presentational purposes, and no significance should be attached to the order of columns. For details of the order in which seats were won at each election, see the detailed results of that election.