South Kasai (French: Sud-Kasaï) was an unrecognised secessionist state within the Republic of the Congo (the modern-day Democratic Republic of the Congo) which was semi-independent between 1960 and 1962. Initially proposed as only a province, South Kasai sought full autonomy in similar circumstances to the much larger neighbouring state of Katanga, to its south, during the political turmoil arising from the independence of the Belgian Congo known as the Congo Crisis. Unlike Katanga, however, South Kasai did not explicitly declare full independence from the Republic of the Congo or reject Congolese sovereignty.

The South Kasaian leader and main advocate, Albert Kalonji, who had represented a faction of the nationalist movement (the Mouvement National Congolais-Kalonji or MNC-K) before decolonisation, exploited ethnic tensions between his own ethnic group, the Baluba, and the Bena Lulua to create a Luba-focused state in the group's traditional heartland in the south-eastern parts of the Kasai region. As sectarian violence broke out across the country, the state declared its secession from the Congo on 9 August 1960 and its government and called for the Baluba living in the rest of the Congo to return to their "homeland". Kalonji was appointed president. Although the South Kasaian government claimed to form an autonomous part of a federal Congo-wide state, it exercised a degree of regional autonomy and even produced its own constitution and postage stamps. The state, supported by foreign powers, particularly Belgium, and funded by diamond exports, managed numerous crises, including those caused by the large emigration of Luba refugees, but became increasingly militarist and repressive.

Soon after its secession, South Kasaian and Congolese troops clashed after the Congolese central government ordered an offensive against it. The resulting campaign, planned to be the first act of a larger action against Katanga, was accompanied by widespread massacres of Baluba and a refugee crisis termed a genocide by some contemporaries. The state was rapidly overrun by Congolese troops. The violence in the suppression of Kasai provided much legitimacy to Joseph Kasa-Vubu's deposition of Patrice Lumumba from the office of prime minister in late 1960 and Lumumba's later arrest and assassination. As a result, South Kasai remained on relatively good terms with the new Congolese government from 1961. Its leaders, including Kalonji himself, served in both the South Kasaian government and the Congolese parliament. South Kasai continued to exercise quasi-independence while Congolese and United Nations troops were able to move through the territory without conflict with the South Kasaian gendarmerie. In April 1961, Kalonji took the royal title Mulopwe ("King of the Baluba") to tie the state more closely to the pre-colonial Luba Empire. The act divided the South Kasaian authorities and Kalonji was disavowed by the majority of South Kasai's parliamentary representatives in Léopoldville. In December 1961, Kalonji was arrested on a legal pretext in Léopoldville and imprisoned, and Ferdinand Kazadi assumed power as acting head of state. UN and Congolese troops occupied South Kasai. In September 1962, shortly after his escape from prison and return to South Kasai, Kalonji was ousted by a military coup d'état which forced him into exile and brought the secession to an end.

The end of South Kasai's secession is usually held to be either December 1961, the date of Kalonji's arrest, or October 1962 with the anti-Kalonji coup d'état and final arrival of government troops.

European colonial rule in the Congo began in the late 19th century. King Leopold II of Belgium, frustrated by his country's lack of international power and prestige, attempted to persuade the Belgian government to support colonial expansion around the then-largely unexplored Congo Basin. The Belgian government's ambivalence about the idea led Leopold to eventually create the colony on his own account. With support from a number of Western countries, who viewed Leopold as a useful buffer between rival colonial powers, Leopold achieved international recognition for a personal colony, the Congo Free State, in 1885. The Luba Empire, the largest regional power in the Kasai region, was annexed into the new state in 1889. By the turn of the century, the violence of Free State officials against indigenous Congolese and the ruthless system of economic extraction had led to intense diplomatic pressure on Belgium to take official control of the country, which it did in 1908, creating the Belgian Congo.

Belgian rule in the Congo was based around the "colonial trinity" (trinité coloniale) of state, missionary and private company interests. The privileging of Belgian commercial interests meant that large amounts of capital flowed into the Congo and that individual regions became specialised. On many occasions, the interests of the government and private enterprise became closely tied and the state helped companies break strikes and remove other barriers imposed by the indigenous population. The country was split into hierarchically-organised administrative subdivisions, and run uniformly according to a set "native policy" (politique indigène) – in contrast to the British and the French, who generally favoured the system of indirect rule whereby traditional leaders were retained in positions of authority under colonial oversight.

Before the start of the colonial period, the region of South Kasai formed part of the Luba Empire, a federation of local kingdoms with a degree of cultural uniformity. During the 17th and 18th centuries, the Baluba spread across large parts of the Kasai-Katanga savannah along the Kasai river basin and eventually developed into a number of ethnic subgroups, notably the Luba-Kasai and the Luba-Katanga. Although never united into a single centralised state, the groups retained a degree of emotional attachment based around shared origin myths and cultural practices. Other groups, like the Songye and the Kanyok, also had long histories in the Kasai region.

One of the major legacies of colonial rule in Kasai was the arbitrary redivision of the population into new ethnic groups. Despite the shared language (Tshiluba) and culture of the two groups, colonial administrators believed the inhabitants of the Lulua river area to be ethnically different from the Baluba and dubbed them the Bena Lulua. The colonists believed the Baluba to be more intelligent, hardworking and open to new ideas than the Bena Lulua. As a result, from the 1930s, the state began to treat the two groups differently and applied different policies to each and promoted the Baluba to positions above other ethnicities.