Balthazar Johannes Vorster (Afrikaans pronunciation: [ˈbaltɑːzar juəˈhanəs ˈfɔrstər]; 13 December 1915 – 10 September 1983), better known as John Vorster, was a South African politician who served as the prime minister of South Africa from 1966 to 1978 and the fourth state president of South Africa from 1978 to 1979. Known as B. J. Vorster during much of his career, he came to prefer the anglicized name John in the 1970s. He was interned in 1942 by the South African government for his involvement in the pro-Nazi Ossewabrandwag.

Vorster strongly adhered to his country's policy of apartheid, overseeing (as Minister of Justice) the Rivonia Trial, in which Nelson Mandela was sentenced to life imprisonment for sabotage, (as Prime Minister) the Terrorism Act, the complete abolition of non-white political representation, the Soweto Riots and the Steve Biko crisis. He conducted a more pragmatic foreign policy than his predecessors, in an effort to improve relations between the white minority government and South Africa's neighbours, particularly after the break-up of the Portuguese colonial empire. Shortly after the 1978 Internal Settlement in Rhodesia, in which he was instrumental, he was implicated in the Muldergate Scandal. He resigned the premiership in favour of the ceremonial state presidency, from which he was forced out as well eight months later.

Vorster was born in 1915 in Jamestown, Cape Province, Union of South Africa, the fifteenth child of a successful sheep farmer, Willem Carel Vorster and his wife, Elizabeth Sophia Vorster (née Wagenaar). He attended primary school there. After Vorster entered Stellenbosch University, he involved himself in student politics becoming the chairman of the debating society, deputy chairman of the student council and leader of the junior National Party.

In 1938, Vorster graduated to become a registrar (judge's clerk) to the judge president of the Cape Provincial Division of the Supreme Court of South Africa but he did not remain in this post for long, setting up his first law practice in Port Elizabeth and his second in the Witwatersrand town of Brakpan.

From 1939, Vorster attracted attention by strongly opposing South Africa's intervention on the side of the Allies and their former foe the United Kingdom, in World War II.

Vorster dedicated himself to an anti-British, pro-Nazi organisation called the Ossewabrandwag (Ox-wagon Sentinel), founded in 1938 in celebration of the centenary of the Great Trek. Under the leadership of Johannes Van Rensburg, the Ossewabrandwag conducted many acts of sabotage against South Africa during World War II to limit its war effort. Vorster, who was interned for his activities, which included helping previously interned fugitives, claimed not to have participated in the acts of war attributed to the group. He described himself as anti-British, not pro-Nazi, and said his internment was for anti-British agitation.

Vorster rose rapidly through the ranks of the Ossewabrandwag becoming a general in its paramilitary wing. In 1942, he said: "We stand for Christian nationalism which is an ally of National Socialism. You can call this anti-democratic principle dictatorship if you wish. In Italy it is called 'Fascism', in Germany 'German National Socialism and in South Africa 'Christian nationalism'."

His involvement with this group led to his detention at Koffiefontein in 1942. Following his release from custody in 1944, Vorster became active in the National Party, which began implementing the policy of apartheid in 1948. Although racial discrimination in favour of whites had long been a crucial fact of South African politics and society, formal restrictions were loosening and the National Party institutionalised racism in a new way and on a massive scale through its “apartheid” legislation.