On the eve of his retirement, Phelps provided an unforgettable reminder of his incredible talent and determination when he came from seventh at the turn to overpower his rivals and win in a time of 51.21 seconds.
South Africa's Chad le Clos, who beat Phelps in the 200 butterfly final, dead-heated for second with Russia's Evgeny Korotyshkin but neither man could hold Phelps off once he started to roll his giant shoulders and kick his powerful feet.
By winning, he joined his American team mate Missy Franklin as the only triple gold medallists in London after she broke the world record in the 200 backstroke final a few minutes earlier and took his career tally to 21 medals, including 17 golds.
He still has one more event to go before he hangs up his goggles, the 4x100 medley relay, an event the US men have never lost at an Olympics they have attended.
