The Union of South Africa (Dutch: Unie van Zuid-Afrika; Afrikaans: Unie van Suid-Afrika pronunciation ) was the historical predecessor to the present-day Republic of South Africa. It came into existence on 31 May 1910 with the unification of the Cape, the Natal, the Transvaal, and the Orange River colonies. It included the territories that were formerly a part of the South African Republic and the Orange Free State.

Following World War I, the Union of South Africa was a signatory of the Treaty of Versailles and became one of the founding members of the League of Nations. It was conferred the administration of South West Africa (now known as Namibia) as a League of Nations mandate. It became treated in most respects as another province of the Union, but it never was formally annexed.

Like Canada, Australia and New Zealand, the Union of South Africa was a self-governing dominion of the British Empire. Its full sovereignty was confirmed with the Balfour Declaration 1926 and the Statute of Westminster 1931. It was governed under a form of constitutional monarchy, with the Crown being represented by a governor-general. The Union came to an end with the enactment of the constitution of 1961, by which it became a republic and left the Commonwealth.