John Duncan may refer to:

• John Duncan (Canadian politician) (born 1948), MP from British Columbia
• John Duncan (footballer), UK football manager
• John Duncan, Sr. (1919–1988), U.S. Representative from Tennessee
• John James Duncan, Jr. (born 1947), U.S. Representative from Tennessee
• John Duncan (US Administration) (born 1945), U.S. Department of the Treasury
• John Duncan (artist), U.S. artist
• John Duncan (musician)
• John Duncan (painter) (1866–1945), Scottish painter
• John Martin Duncan (1851–1917), Texas state senator

• Ross Duncan (born 1944), Australian cricketer, given name John