Adam Scott Mathieson Duncan (2 November 1888 – 3 October 1976) was a Scottish football player and manager, who played as an outside right. He made over 100 appearances for Rangers, played in England for Newcastle United, winning the Football League title in 1908–09. He also had three spells with Dumbarton, and played for Cowdenbeath and guested for Celtic. As a manager, he led Hamilton Academical and Cowdenbeath before going to England, where he managed Manchester United and then Ipswich Town, the latter for 18 years.