Lanark South was a federal electoral district represented in the Canadian House of Commons from 1867 to 1917. It was located in the province of Ontario. It was created by the British North America Act of 1867 which divided the County of Lanark into two ridings: Lanark South and Lanark North. 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.