The Kasai River (called Cassai in Angola) is a tributary of the Congo River, located in central Africa. The river begins in Angola and serves as the border between Angola and the Democratic Republic of the Congo (DRC), then flows into the DRC, where it joins the Congo northeast of Kinshasa. The Kasai's tributaries include the Fimi, Kwango, and Sankuru rivers. The short stretch of the Kasai from the inflow of the Fimi to the Congo is known as the Kwa River.
The Kasai watershed consists mainly of equatorial rainforest areas.