At the 2003 World Aquatics Championships in Barcelona, Spain, Peirsol won gold in the 100 and 200-meter backstroke and the 4×100-meter medley relay. He also won a silver medal in the 4×200-meter freestyle relay. In his first event, the 100-meter backstroke, Peirsol won gold in a time of 53.61, a championship record. His time was just 0.01 s off Lenny Krayzelburg's world record. Peirsol participated in the 4×200-meter freestyle relay with Michael Phelps, Nate Dusing, and Klete Keller to win silver behind Australia. Peirsol then competed in the 200-meter backstroke and won gold in a time of 1:55.92, almost two seconds ahead of second-place finisher Gordan Kožulj of Croatia. Peirsol also competed in the 50-meter backstroke and the 4×100-meter medley relay. In the 50-meter backstroke, Peirsol finished out of medal contention with an 8th-place finish. In his last event, Peirsol competed in the 4×100-meter medley relay with Brendan Hansen, Ian Crocker, and Jason Lezak. The final time of 3:31.54 was a new world record, bettering the old mark from 2002.