Thomas Allen Shields (born July 11, 1991) is an American competitive swimmer. He is a two-time Olympian and the current American record-holder in the short course 50-meter butterfly and 200-meter butterfly. At the 2016 Summer Olympics, he won a gold medal in the 4×100-meter medley relay, swimming the butterfly leg of the relay in the prelims. Later the same year, he became the first American to swim faster than 44 seconds in the 100-yard butterfly, setting a new American record of 43.84 seconds.

For the 2020 Summer Olympics, he swam the butterfly leg of the 4×100-meter medley relay in the prelims, winning a gold medal when the finals relay finished first. At the Kazan stop of the 2021 FINA Swimming World Cup, Shields was the only swimmer representing the United States to compete, winning two gold medals and one silver medal in individual events. For the whole 2021 World Cup, Shields was the highest scoring American competitor and second-highest scoring male competitor.

Shields was born in Panama City, Florida, while his father was stationed at Tyndall Air Force Base. When he was two years old, his parents moved him and his older sister, Lindy, to Huntington Beach, California. Once in Orange County, he began swimming competitively when he was eight years old.

For high school, Shields attended Edison High School where he swam on the Varsity Swim Team all four years, receiving multiple awards, breaking the National High School record in the 200 yard freestyle with a time of 1:33.83, and ending his high school career with an induction into the Edison High School Hall of Fame in 2009. He attended the University of California, Berkeley from 2010 to 2013, where he swam under Head Coach Dave Durden and graduated with an undergraduate degree in religious studies in 2013.

At the 2008 U.S. Olympic Team Trials in Omaha, Nebraska, Shields placed 45th in the 200-meter freestyle with a time of 1:51.74, 54th in the 400-meter freestyle at 4:00.83, 72nd in the 200-meter butterfly with a time of 2:04.78, and 89th in the 100-meter butterfly in 55.77 seconds.

At the 2010 NCAA Division I Championships, he won the 100-yard butterfly, placed second in the 200-yard butterfly, and was 16th in the 200-yard freestyle. Both of his butterfly efforts broke 17-18 National Age Group records setting new records of 44.91 in the 100-yard butterfly and 1:41.59 in the 200-yard butterfly. He was on the winning 4x100-yard medley relay, and the winning 4x100-yard freestyle relay. He helped his university team, the California Golden Bears, finish second to the Texas Longhorns in the championships. For his performances in the NCAA his freshman year, Shields won the "Freshman/Newcomer of the Year" award from the Pac-12 Conference.

At the 2011 NCAA Division I Championships, Shields won the 100-yard backstroke in 45.02 seconds, finished in second place in the 100-yard butterfly at 44.91 seconds, and placed third in the 200-yard butterfly with a time of 1:41.20. He was the first champion from the California Golden Bears in the 100-yard backstroke at an NCAA Division I Swimming and Diving Championships. He anchored the second place 4×50-yard freestyle relay, the winning 4×100-yard freestyle relay, and the second place in 4×200-yard freestyle relay. For the medley relays, Shields swam the butterfly leg of each relay. His team won the championships, narrowly defeating the Texas Longhorns who won the championships the year before.

At the 2011 Summer Universiade in Shenzhen, Shields earned the first international medal of his career, a gold, in the 4×200-meter freestyle relay for his contribution in the heats. Shields earned his first individual medal, a silver, in the 100-meter butterfly, finishing behind compatriot Tim Phillips. Shields also earned a silver medal in the 4×100-meter medley relay for his contribution in the heats.