Lake Tanganyika (/ˌtæŋɡənˈjiːkə, -ɡæn-/ TANG-gən-YEE-kə, -⁠gan-; Kirundi: Ikiyaga ca Tanganyika) is an African Great Lake. It is the world's second-largest freshwater lake by volume and the second deepest, in both cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. It is also the 6th largest lake by area. The lake is shared among four countries—Tanzania, the Democratic Republic of the Congo (the DRC), Burundi, and Zambia—with Tanzania (46%) and the DRC (40%) possessing the majority of the lake. It drains via the Lukuga River into the Congo River system, which ultimately discharges at Banana, Democratic Republic of the Congo into the Atlantic Ocean.

Lake Tanganyika is situated within the Albertine Rift, the western branch of the East African Rift, and is confined by the mountainous walls of the valley. It is the largest rift lake in Africa and the second-largest freshwater lake by volume in the world. It is the deepest lake in Africa and holds the greatest volume of fresh water on the continent, accounting for 16% of the world's available fresh water. It extends for 676 km (420 mi) in a general north–south direction and averages 50 km (31 mi) in width. The lake covers 32,000 km² (12,000 sq mi), with a shoreline of 1,900 km (1,200 mi), a mean depth of 572 m (1,877 ft) and a maximum depth of 1,471 m (4,826 ft) (in the northern basin). It holds an estimated 18,750 km³ (4,500 cu mi) of water.

The catchment area of the lake is 231,000 km² (89,000 sq mi). Two main rivers flow into the lake, as well as numerous smaller rivers and streams (whose lengths are limited by the steep mountains around the lake). The one major outflow is the Lukuga River, which empties into the Congo River drainage. Precipitation and evaporation play a greater role than the rivers. At least 90% of the water influx is from rain falling on the lake's surface and at least 90% of the water loss is from direct evaporation.

The major river flowing into the lake is the Ruzizi River, formed about 10,000 years ago, which enters the north of the lake from Lake Kivu. The Malagarasi River, which is Tanzania's second largest river, enters the east side of Lake Tanganyika. The Malagarasi is older than Lake Tanganyika, and before the lake was formed, it probably was a headwater of the Lualaba River, the main Congo River headstream.

The lake has a complex history of changing flow patterns, due to its high altitude, great depth, slow rate of refill, and mountainous location in a turbulently volcanic area that has undergone climate changes. Apparently, it has rarely in the past had an outflow to the sea. It has been described as "practically endorheic" for this reason. The lake's connection to the sea is dependent on a high water level allowing water to overflow out of the lake through the Lukuga River into the Congo. When not overflowing, the lake's exit into the Lukuga River typically is blocked by sand bars and masses of weed, and instead this river depends on its own tributaries, especially the Niemba River, to maintain a flow.

The lake may also have at times had different inflows and outflows; inward flows from a higher Lake Rukwa, access to Lake Malawi and an exit route to the Nile have all been proposed to have existed at some point in the lake's history.

Lake Tanganyika is an ancient lake, one of only twenty more than a million years old. Its three basins, which in periods with much lower water levels were separate lakes, are of different ages. The central began to form 9–12 million years ago (Mya), the northern 7–8 Mya and the southern 2–4 Mya.

The lake's water is alkaline with a pH around 9 at depths of 0–100 m (0–330 ft). Below this, it is around 8.7, gradually decreasing to 8.3–8.5 in the deepest parts of Tanganyika. A similar pattern can be seen in the electric conductivity, ranging from about 670 μS/cm in the upper part to 690 μS/cm in the deepest.