The American swimmer who broke several world records and dominated the sport in the early 2000s is Michael Phelps. Known as the most decorated Olympian of all time, he has won 23 gold medals and set numerous world records throughout his career. Phelps' swimming prowess and versatility across different events, such as the butterfly and freestyle, led to his remarkable success at multiple Olympics. His dedication to training and performance continues to inspire many athletes worldwide and has significantly impacted the sport of swimming.