The Rufiji River lies entirely within Tanzania. It is also the largest and longest river in the country. The river is formed by the confluence of the Kilombero and Luwegu rivers. It is approximately 600 kilometres (370 mi) long, with its source in southwestern Tanzania and its mouth on the Indian Ocean opposite Mafia Island across the Mafia Channel, in Pwani Region. Its principal tributary is the Great Ruaha River. It is navigable for approximately 100 kilometres (62 mi).

The Rufiji river is approximately 200 kilometres (120 mi) south of Dar es Salaam. The river's delta contains the largest mangrove forest in eastern Africa.^[1]

A branch of ancient sea routes led down the East African coast called "Azania" by the Greeks and Romans in the 1st century CE as described in the Periplus of the Erythraean Sea (and, very probably, Chinese: 澤散 in the 3rd century by the Chinese),^[2] at least as far as the port known to the Romans as Rhapta, which was probably located in the delta of the Rufiji River in modern Tanzania.^[3]

During the First World War, from October 1914 to July 1915, the river delta was the scene of a protracted naval operation. These were the attempts, and later achievement, by the Royal Navy to neutralize and destroy the German cruiser Konigsberg.

The catchment basin for the Rufiji River complex is 177,429 square kilometres (68,506 sq mi).^[4]

Tanzania president John Magufuli has approved the construction of a controversial^[5][6] new dam and power station on the river at Stiegler's Gorge.^[7] The power station is expected to provide 2,100 megawatts of electricity, more than triple Tanzania's existing installed hydropower capacity which is only 562 megawatts.^[8] Construction of the dam started on July 26, 2019, and it is expected be ready by 2022.^[9]