South Africa became a republic on May 31, 1961, the result of the Republic of South Africa Constitution Act of 1961. Previously, South Africa had been a dominion of the Commonwealth (formerly the British Commonwealth of Nations).
The modern day country was first established as the Union of South Africa, which unified the British colonies of the Cape Colony, Natal, Transvaal, and Orange River (later Orange Free State), and was inaugurated on May 31, 1910. It was a self-governing dominion within the British Empire. The United Kingdom’s Statute of Westminster in 1931, which codified previous decisions made in 1926 and 1930, made South Africa a dominion of the British Commonwealth of Nations, later known as the Commonwealth, and South Africa passed the related Status of the Union Act in 1934 affirming its sovereign independent status. In October 1960, a proposal for South Africa to become a republic was put to South Africans in a referendum (notably, only white South Africans voted, as this was during the apartheid era of racial segregation and discrimination); it was approved by a simple majority. When South Africa became a republic in 1961, it also left the Commonwealth. (The country would later rejoin the organization in 1994, after the fall of apartheid).