South Brother, also known as Île du Sud, is a 23 ha coral island on the Great Chagos Bank atoll of the Chagos Archipelago in the British Indian Ocean Territory. It is one of the three islands in the Three Brothers group on the western side of the atoll, and forms part of the Chagos Archipelago strict nature reserve. It has been identified as an Important Bird Area by BirdLife International because of its significance as a breeding site for seabirds, including brown noddies (with 6100 breeding pairs recorded in a 2004 survey) and lesser noddies (7300 pairs).