Sir John William Downer, KCMG, KC was an Australian politician who served two terms as Premier of South Australia, from 1885 to 1887 and again from 1892 to 1893. He later entered federal politics and served as a Senator for South Australia from 1901 to 1903. He was the first of four Australian politicians from the Downer family dynasty.

The premier of South Australia is the head of government in the state of South Australia, Australia. The Government of South Australia follows the Westminster system, with a Parliament of South Australia acting as the legislature. The premier is appointed by the governor of South Australia, and by modern convention holds office by virtue of their ability to command the support of a majority of members of the lower house of Parliament, the House of Assembly.