Sir William John Lyne KCMG (6 April 1844 – 3 August 1913) was an Australian politician who served as Premier of New South Wales from 1899 to 1901, and later as a federal cabinet minister under Edmund Barton and Alfred Deakin. He is best known as the subject of the so called "Hopetoun Blunder", unexpectedly being asked to serve as the first Prime Minister of Australia but proving unable to form a government.