The Honourable Sir William Macarthur (December 1800 – 29 October 1882) was an Australian botanist and vigneron. He was one of the most active and influential horticulturists in Australia in the mid-to-late 19th century. Among the first viticulturists in Australia, Macarthur was a medal-winning wine-maker, as well as a respected amateur botanist and noted plant breeder.