The Sudd (Arabic sadd, "barrier") is a vast swamp formed by the Nile in southern Sudan. Its size is variable, but during the wet season it may be over 130,000 km² in area.

In the Sudd, the river flows through multiple tangled channels in a pattern that changes each year. Papyrus grows in dense thickets in the shallow water, which is frequented by crocodiles and hippopotami. Sometimes the matted vegetation will build up into an island, but these eventually break up.

There are three main waterways through the swamp; the Bahr-el-Zaraf ("River of the Giraffes"), the Bahr el Ghazal ("River of the Gazelles"), and the Bahr al Jabal, which is the main connection to the Mountain Nile. The early explorers searching for the source of the Nile experienced considerable difficulties, sometimes taking months to get through.

In The White Nile , Alan Moorehead says of the Sudd, "there is no more formidable swamp in the world".