Marais Viljoen (2 December 1915 – 4 January 2007) was the last ceremonial State President of South Africa from 4 June 1979 until 3 September 1984. Viljoen became the last of the ceremonial presidents of South Africa when he was succeeded in 1984 by an executive president, P. W. Botha.

Viljoen was the youngest of six children. He was married on 20 April 1940 to Dorothea Maria Brink (17 September 1917 – 5 October 2005), with whom he had one daughter Elizabeth Magdalena (Elna) Viljoen.

Before entering a political career, he worked in the Post Office, and thereafter at the Afrikaans language newspaper, Die Transvaler, edited by Hendrik Verwoerd, who later became Prime Minister.

Viljoen was elected to the House of Assembly as MP for Alberton, near Johannesburg, as President of the Senate, and as acting State President from 21 August 1978 to 10 October 1978, after which B.J. Vorster was briefly elected to the position. Viljoen was seen as a relatively moderate member of the National Party that instituted apartheid.