Cape Breton South (French: Cap-Breton-Sud) was a federal electoral district in the province of Nova Scotia, Canada, that was represented in the House of Commons of Canada from 1904 to 1911 and from 1925 to 1968.

It consisted of the southern part of the county of Cape Breton, i.e., the districts of Balls Creek Bateston, Big Pond, Bridgeport, Catalone, Dominion No.

1 and Reserve Mines, Fast Bay (South), Gabarus, Grand Mira, Hillside, Loch Lomond, Louisbourg, Main-à-Dieu, Port Morien, Sydney Forks, Trout Creek, Victoria Mines and Lingan, and the towns of Glace Bay, Louisbourg and Sydney.

It 1933, it was redefined to consist of the part of the county of Cape Breton contained in the municipal districts of Dominion No.

18), and including the city of Sydney and the towns of Glace Bay, New Waterford and Dominion.