Ian Crocker (born August 31, 1982 in Portland, Maine) is a former American swimmer. He is a three time Olympic champion and former world record holder in the 100 m butterfly.
Crocker, who specializes in the butterfly, has received five Olympic medals in his career. In addition to his accomplishments at the Olympics, Crocker was the first man to ever swim under 51 seconds in the 100 meter butterfly. Crocker improved his world record of 50.98 twice, down to a time of 50.40 set at the 2005 World Aquatics Championships in Montreal, before being bested by Michael Phelps in 2009. Crocker also won two golds and one silver at the 2003 World Aquatics Championships, and two golds and one silver at the 2005 World Aquatics Championships. At the 2007 World Aquatics Championships in Melbourne, Australia, Crocker, who had been in position to derail Phelps in the 100 fly before losing to his rival, dove in too early on an exchange, causing the disqualification of the American team's 4x100 m medley relay.
Crocker swam in two swimming events at the 2000 Summer Games.