Michael Fred Phelps II (born June 30, 1985) is a retired American competitive swimmer who is widely regarded as the most successful and decorated Olympian of all time. Over the course of his career, Phelps won a total of 28 Olympic medals, including 23 gold, setting records for the most Olympic gold medals, the most gold medals in individual events (13), and the most medals in individual events overall (16).

At the 2004 Athens Olympics, Phelps equaled gymnast Alexander Dityatin's record by winning eight medals in a single Games—six gold and two bronze. He surpassed this achievement at the 2008 Beijing Olympics, where he claimed eight gold medals, breaking fellow American swimmer Mark Spitz's 1972 record of seven golds in a single Olympic Games. Phelps continued his dominance at the 2012 London Olympics, winning four gold and two silver medals, and at the 2016 Rio de Janeiro Olympics, he earned five gold and one silver medal. This made him the most successful athlete at four consecutive Olympic Games.

Phelps also held world records in several long course events, including the 200-meter freestyle, 100-meter and 200-meter butterfly, and the 200-meter and 400-meter individual medley. He accumulated 82 medals in major international competitions—65 gold, 14 silver, and three bronze—across the Olympics, World Championships, and Pan Pacific Championships. His record-breaking performances earned him numerous accolades, including the World Swimmer of the Year Award eight times, the American Swimmer of the Year Award eleven times, and the FINA Swimmer of the Year Award in 2012 and 2016. In recognition of his extraordinary achievements at the 2008 Olympics, Phelps was named Sports Illustrated's Sportsman of the Year.

Following the 2008 Olympics, Phelps established the Michael Phelps Foundation, dedicated to promoting swimming and healthier lifestyles. Although he initially retired after the 2012 Olympics, Phelps made a comeback in 2014. At his fifth Olympics in Rio in 2016, he was chosen as the United States flag bearer for the Parade of Nations. After these Games, Phelps announced his final retirement on August 12, 2016, having won more Olympic medals than 161 countries. He was honored with the Laureus World Comeback of the Year Award in 2017.

Phelps is widely recognized as the greatest swimmer of all time and is frequently considered one of the greatest athletes in history.