Alexander Duncan may refer to:

• Alexander Duncan (army officer) (1780–1859), East India Company officer
• Alexander Duncan (athlete) (1884–1959), British Olympic runner
• Alexander Duncan (bishop) (1655–1733), Scottish Episcopal clergyman and bishop
• Alexander Duncan (politician) (1788–1853), U.S. Representative from Ohio
• Alexander Duncan (police officer) (1888–1965), Scottish police officer and Chief Commissioner of Victoria Police (Australia), 1937–1954
• Alex Duncan (1900–1984), former Australian rules footballer
• Alexander Duncan (doctor), British doctor who spent six years in a village high up in the Wakhan Corridor of north-eastern Afghanistan
• Alexander William Duncan (1881–1934), Scottish cricketer and rugby player
• Alexander Duncan (footballer) (1891–?), Scottish footballer

• All pages with titles beginning with Alexander Duncan