Aaron Wells Peirsol (born July 23, 1983 in Irvine, California) is an American competitive swimmer. He is best known for winning both available gold medals for men in the backstroke at the 2004 Summer Olympics in Athens, Greece. After winning gold in the 100 m backstroke, he followed up by winning the 200 m event, but an initial disqualification alleged that he had made an illegal turn during the race. Having been improperly entered, the disqualification was overturned, giving Peirsol his second individual gold. Peirsol won the world championships in the 100 m backstroke in 2003, 2005 and 2007, and won 200 m backstroke in 2001, 2003 and 2005.

He also holds the world record for the 100 m backstroke and has been undefeated in finals in the 100 m since the 2002 Spring Nationals. He was also undefeated in the 200 m since losing to Lenny Krayzelburg in the 2000 Sydney Olympics until being defeated by countryman Ryan Lochte at the 2007 World Championships.

Peirsol is a student-athlete at the University of Texas, although he considers Newport Beach, California home. He trains with fellow world record holders and gold medalists Ian Crocker, Brendan Hansen, and Neil Walker under the guidance of Eddie Reese. Aaron is also known for his diversity in events other than backstroke such as butterfly and middle distance freestyle.

His younger sister, Hayley Peirsol, is a top swimmer at Auburn University.

At the 2007 World Championships, Peirsol lowered his own 100 m Backstroke record to 52.98, and in doing so became the first man under 53 seconds. At one point during the first 50 metersPeirsol was a full meter behind Ryan Lochte, but Peirsol's final 25 meters was enough to over-take Lochte at the finish.