The Congo Basin (French: Bassin du Congo) is the sedimentary basin of the Congo River. The Congo Basin is located in Central Africa, in a region known as west equatorial Africa. The Congo Basin region is sometimes known simply as the Congo. It contains some of the largest tropical rainforests in the world and is an important source of water used in agriculture and energy generation.

The rainforest in the Congo Basin is the largest rainforest in Africa and second only to the Amazon rainforest in size, with 300 million hectares compared to the 800 million hectares in the Amazon. Because of its size and diversity the basin's forest is important for mitigating climate change in its role as a carbon sink. However, deforestation and degradation of the ecology by the impacts of climate change may increase stress on the forest ecosystem, in turn making the hydrology of the basin more variable. A 2012 study found that the variability in precipitation caused by climate change will negatively affect economic activity in the basin.

Eight sites of the Congo Basin are inscribed on the World Heritage List, five being also on the list of World Heritage in Danger (all five located in Democratic Republic of the Congo). Fourteen percent of the humid forest is designated as protected.

The Congo Basin is a large depression within the Congo Craton, making it a patch of relatively recent (Phanerozoic-aged, and mostly Mesozoic & onwards) sedimentary rock within a large, otherwise extremely ancient (Archean-aged) piece of exposed continental crust. The deformation of the Craton began as early as the late Cambrian or early Ordovician and continued over the Paleozoic, but the deformation over this period led to rapid erosion of much of this Paleozoic rock, creating a large unconformity. Sediment started to rapidly accumulate in the basin from the Mesozoic (Triassic) up to the present day.

Deposits throughout the Jurassic suggest the presence of a freshwater, lacustrine habitat in the basin, and this continued into the Early Cretaceous. By the start of the Late Cretaceous, a connection with the Trans-Saharan seaway led to a significant marine incursion into the basin (evidence of an earlier, Late Jurassic marine intrusion is disputed), causing it to serve as a connection between the southern Atlantic Ocean and the Tethys Ocean. Many of the formations deposited by these freshwater and marine habitats are rich in pollen, invertebrate, and vertebrate (primarily fish) fossils. Kimberlite pipes that are thought to have formed during the Cretaceous, possibly due to a shock from a sudden decrease in the rate of seafloor spreading of the southern Mid-Atlantic Ridge, are the source of the region's famous diamonds.

By the Cenozoic, an uplift in the borders of the Cuvette Centrale had blocked any further marine connections. During the Paleogene, high rainfall turned the basin into a series of marshy ponds and swamps. A shift to more arid conditions with seasonal droughts occurred with the start of the Neogene. Later in the Neogene, a sudden shift to fluvial deposits suggests a dramatic return to wetter conditions.

The following sedimentary geological formations have been deposited in the basin:

Congo is a traditional name for the equatorial Middle Africa that lies between the Gulf of Guinea and the African Great Lakes. The basin begins in the highlands of the East African Rift system with input from the Chambeshi, the Uele and Ubangi rivers in the upper reaches and the Lualaba River draining wetlands in the middle reaches. Because of the young age and active uplift of the East African Rift at the headwaters, the river's yearly sediment load is very large, but the drainage basin occupies large areas of low relief throughout much of its area. It is delineated largely by swells including the Bie, Mayumbe, Adamlia, Nile-Congo, East African, and Zambian Swells.