Mari Hemmer /2maːri 1hemer/ (born November 20, 1983) is a Norwegian long-track speedskater, with particular strength in the longer distances 3000-m and 5000-m. She is the Norwegian Allround Champion for 2005, 2007, 2008, 2010, the Norwegian Sprint Champion for 2005, 2009, 2010, and has also won numerous gold, silver, and bronze medals from national championships at single distances. Along with fellow skaters Maren Haugli and Hege Bøkko, Hemmer has in recent years represented Norway at the European Championships, the World Allround Championships and the World Single Distance Championships, as well as in the World Cup.
As of April 2011, Hemmer is the 3rd best ever Norwegian female skater, as per the Adelskalenderen, after Maren Haugli and Ida Njåtun. Her rank on the international Adelskalenderen, as of after the 2010-2011 season, is 46th place.
In March 2009, Hemmer broke her own national record in the 10,000-m, improving the time from 14:50.52 to 14:47.49. This feat was carried out just some hours after having skated the 5,000-m in 7:13.50, making her one of the few skaters ever who have run both the 5,000-m and 10,000-m on the same day.