Kasaï-Occidental (French for "Western Kasai"; Luba-Kasai: Kasai Wa Mubuelu) was one of the eleven provinces of the Democratic Republic of the Congo between 1966 and 2015, when it was split into the Kasaï-Central and the Kasaï provinces.

The province of Kasaï-Occidental was established in 1966 by regrouping the provinces of Luluabourg and Unité Kasaïenne which in turn were created 1962 when the historical Kasaï Province was divided in five provinces namely Lomami, Sankuru, Sud-Kasai, Luluabourg, Unité-Kasaïenne. The former provinces of Luluabourg and Unité-Kasaïenne correspond to the current districts of Lulua District and Kasaï District. Since its formation the provincial seat is Kananga (formerly Luluabourg) which was also the seat of the Kasaï Province between 1957 and 1962. The Province of Lusambo precedes the current entity, it was created 1933 by carving out the districts of Kasai and Sankuru from the Province of Congo-Kasaï one of the four provinces established in 1924. The province of Lusambo changed its name to Kasaï Province in 1947 its provincial seat was still Lusambo until 1957 when it was moved to Luluabourg (now Kananga). The Province of Kasai existed until it was subdivided in 1962 into 5 provinces.

The Province derives it name from the Kasaï river which flows through the Kasaï District from south to north. The river is the second longest in DR. Congo and a major tributary of the Congo river. The Kasaï river was variously called Enzzadi, Nsadi, Nzadi, Kassabi, Kasye, Kassaba by the different local tribes (Luba, Tshokwe, Lunda, etc..), latter European explorers came to use the name Kasaï to refer to the river.

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