The Kasai River is a river in central Africa. The river begins in Angola. Part of the river serves as the border between Angola and the Democratic Republic of the Congo (DRC). It then flows into the DRC, where it empties into the Congo River.

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.