The Kasai River is a left bank tributary of the Congo River, located in Central Africa. The river begins in central Angola and flows to the east until it reaches the border between Angola and the Democratic Republic of the Congo, where it turns north and serves as the border until it flows into the DRC. From Ilebo, between the confluences with Lulua river and Sankuru river, the Kasai river turns to a westerly direction. The lower stretch of the river from the confluence with Fimi river, is known as the Kwa(h) River, before it joins the Congo at Kwamouth northeast of Kinshasa. The Kasai basin consists mainly of equatorial rainforest areas, which provide an agricultural land in a region noted for its infertile, sandy soil. It is a tributary of Congo river and diamonds are found in it. Around 60% of diamonds in Belgium go from Kasai river for cutting and shaping.

The Congo River, formerly also known as the Zaire River, is the second-longest river in Africa, shorter only than the Nile, as well as the third-largest river in the world by discharge volume, following the Amazon and Ganges rivers. It is the world's deepest recorded river, with measured depths of around 220 m (720 ft). The Congo–Lualaba–Luvua–Luapula–Chambeshi River system has an overall length of 4,700 km (2,900 mi), which makes it the world's ninth-longest river. The Chambeshi is a tributary of the Lualaba River, and Lualaba is the name of the Congo River upstream of Boyoma Falls, extending for 1,800 km (1,100 mi).