The King or Queen of South Africa was the nominal ruler of the Union of South Africa during the state's existence as a dominion of the British Empire (and later a realm of the British Commonwealth) from May 31 1910 to May 31 1961 when the country became the Republic of South Africa. Since the Union of South Africa was a British dominion the British monarch served as South Africa's monarch represented by the Governor-General of the Union of South Africa.

Until the coronation of Elizabeth II, British monarchs did not have separate titles as King (or Queen) of South Africa but were formally (after 1927) King of the United Kingdom of Great Britain and Northern Ireland and the British dominions beyond the seas. With Elizabeth II's ascension occurring soon after the transformation of the British Empire into the Commonwealth of Nations, dominions began to give the monarch separate titles and thus Elizabeth had the title of Queen of South Africa while her predecessors were kings of South Africa in law but not in title.

Prior to 1927 the King was considered "King in South Africa" rather than "King of South Africa" as until the passage of the 1927 Royal Style and Titles Act British constitutional law held that there was a single individisble monarchy with the dominions treated as subsets of the United Kingdom. After 1927, and particularly after the Statute of Westminster, 1931 the crowns of the various dominions were conidered as separate entities though with a shared throne.

In 1961 South Africa became a republic.

Kings and Queens of the Union of South Africa

• George V (1910-1936)
• Edward VIII (1936)
• George VI (1936-1952)
• Elizabeth II (1952-1961)