Lanark South was a federal electoral district represented in the House of Commons of Canada from 1867 to 1917.

In 1882, the South Riding of Lanark was defined as consisting of the townships of Bathurst, North Elmsley, Beckwith, South Sherbrooke, North Burgess, Drummond and Montague, the Town of Perth, and the Village of Carleton Place.

In 1903, the Town of Smiths Falls was added to the riding, and the Village of Carleton Place was excluded.

The electoral district was abolished in 1914 when it was merged into Lanark riding.

This riding has elected the following members of Parliament: On Mr. Morris being named Minister of Inland Revenue, 16 November 1869: On Mr. Haggart being named Postmaster General, 3 August 1888: On Mr. Haggart's death, 13 March 1913: