Martin Johnsrud Sundby (born 26 September 1984) is a Norwegian cross-country skier who has competed since 2003. He is the silver medalist of the 2010 Winter Olympics in the team relay and the bronze medalist of the 2014 Winter Olympics in the 30 km skiathlon. He is the first Norwegian to have won the Tour de Ski.
Sundby got his international breakthrough when he won his first individual victory on 30 November 2008 in Kuusamo, Finland. The victory was regarded as a major surprise, despite Sundby delivering several solid displays in earlier races. At lower levels, Sundby has won a junior sprint event in 2003, a 30 km Scandinavian Cup race in 2007, and a 10 km FIS race in 2007. He later won the team sprint event at the test event in Liberec, Czech Republic on 17 February 2008.
Sundby and compatriot Therese Johaug became the first Norwegians to win the Tour de Ski when they won the men's and women' competitions in the 2013-14 edition of the race. Sundby subsequently won the overall and distance competitions in the 2013–14 FIS Cross-Country World Cup. He also won the 2016 Tour de Ski. In addition to his Tour de Ski victories, Sundby won the inaugural Ski Tour Canada, despite starting behind Sergey Ustiugov and Petter Northug on the final stage.