(Photo credit: Al Bello/Getty Images)
Peirsol, the defending champion and world record holder, qualified fifth fastest in 53.56 for the final but showed his class in swimming down Liam Tancock of Great Britain who turned in front at the 50m 0.49 seconds under the world record mark.
Peirsol surged ahead to win comfortably in 52.54 to beat the world record of 52.89 he set in July.
After qualifying second fastest for the final, Peirsol's USA teammate Matt Grevers finished with the silver medal in 53.11.
Hayden Stoeckel of Australia, who qualified fastest for the final in an Olympic record 52.97, deadheated with Russia's Arkady Vyatchanin for the bronze medal in 53.18.
Tancock faded badly in the final 50m to finish sixth.
(Photo credit: Al Bello/Getty Images)
