The South Africa Act 1909 (9 Edw. 7. c. 9) was an act of the Parliament of the United Kingdom that created the Union of South Africa out of the former Cape, Natal, Orange River, and Transvaal colonies, and served as the constitution of the Union from 1910 until 1961. The act also allowed for potential admission of Rhodesia into the Union, a proposal rejected by Rhodesian colonists in a 1922 referendum. The draft proposal was supported by the four colonial parliaments, but was opposed by Cape Colony premier W. P. Schreiner, who raised concerns that it would strip rights from non-white South Africans.
The act was the third major piece of legislation passed by the British Parliament to unite various British colonies and provide some degree of autonomy. This had been done through the British North America Act 1867, which united the Province of Canada (split into Ontario and Quebec) with Nova Scotia and New Brunswick, and the unification of Australia through the Commonwealth of Australia Constitution Act 1900.