Derek Mandell (born in Tamuning, Guam) is a Guamanian middle-distance runner.
He took part in the 2008 Oceania Athletics Championships, winning bronze, and 2008 Summer Olympics, where he broke a personal men's 800m record even though Mandell was eliminated in the first round.
He later competed in the 2010 Guam Athletics Championships, where he won two gold medals, the 2011 Pacific Games, where he won a silver medal, and the 2011 World Championships in Athletics, where he set a personal best time of 1:57.11.
He also competed in the men's 800m event at the 2012 Summer Olympics but was again eliminated in the first round.