South Louth was a parliamentary constituency in Ireland, which returned one Member of Parliament (MP) to the House of Commons of the Parliament of the United Kingdom, elected on a system of first-past-the-post, from 1885 to 1918. Prior to the 1885 general election and after the dissolution of Parliament in 1918 the area was part of the Louth constituency.

This constituency comprised the southern part of County Louth including the towns of Drogheda and Ardee. The seat was defined under the Redistribution of Seats Act 1885 as comprising the baronies of Drogheda and Ferrard, that part of the barony of Ardee not contained within the constituency of North Louth, and the county of the town of Drogheda.

The elections in this constituency took place using the first past the post electoral system.