Sir John William Downer, KCMG, KC (6 July 1843 – 2 August 1915) 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.