William Duncan may refer to:
William Barr McKinnon Duncan (1922–1984), director of ICI, chief executive of Rolls Royce
William fitz Duncan (died 1147), Scottish prince and general
William Duncan (missionary) (1832–1918), English-born Anglican missionary
William Addison Duncan (1836–1884), U.S. Representative from Pennsylvania
William Garnett Duncan (1800–1875), U.S. Representative from Kentucky
William Henry Duncan (1805–1863), English doctor and Britain's first Chief Medical Officer
William Wallace Duncan (1839–1908), American bishop of the Methodist Episcopal Church, South, elected in 1886
William Duncan (actor) (1879–1961), actor and director of film serials
William Dow Duncan (1892–1961), New Zealand rugby player
Art Duncan (William James Arthur Duncan, 1891–1975), World War I flying ace
William Duncan (footballer) (1913–?), Scottish association football player
William Duncan (philosopher) (1717–1760), Scottish natural philosopher and classicist
William Butler Duncan (1853–1912), New York City banker
William Augustine Duncan (1811–1885), Scottish journalist and colonial official
William C. Duncan (1820–1877), brewer, politician, and mayor of Detroit, Michigan
W Murdoch Duncan (William Murdoch Duncan, 1909–1976), crime thriller author
Sir William Duncan, 1st Baronet (died 1774), Scottish physician