Mark William Geiger (born August 25, 1974) is an American sports administrator and former soccer referee. He is the senior director of match officials at the Professional Referee Organization (PRO), which oversees domestic referees in Major League Soccer (MLS). Geiger previously officiated in MLS and was on the FIFA International Referees List from 2008 to 2019. At the international level, Geiger refereed the 2012 Olympics; at the 2013, 2015 and 2017 CONCACAF Gold Cups; and at the 2014 and 2018 FIFA World Cups. At the 2014 World Cup, he became the first referee from the United States to officiate a knockout match at a World Cup tournament. Before becoming a full-time referee, Geiger was a high school mathematics teacher.