Marcel Siem (born 15 July 1980) is a German professional golfer.
Siem was born in Mettmann. He turned professional in 2000. He has been a member of the European Tour since 2002 and his first and only win on the Tour to date came at the 2004 Dunhill Championship. His best year-end ranking on the Order of Merit is 39th in 2004.
Siem represented Germany at the WGC-World Cup in 2003, 2004, and 2006. In 2006 he was Bernhard Langer's team-mate in the first winning German team in the World Cup.