Reaching the midway point of the 100-meter butterfly in next-to-last place, he cruised past all his rivals on the way home and won with a little bit to spare.
Phelps, who has stood on the Olympic victory podium more times than any other athlete in history, won his 17th gold medal and his 21st medal overall.
Until these Olympics, no one had won the same swimming event in three consecutive Games. Phelps has now done so -- twice. His final Olympic swim will come Saturday in the 400-meter medley relay.
His victory was one of three for the Americans in the pool Friday, the other two coming in stunning performances by young women who stand ready to carry on Phelps' legacy of excellence.
Missy Franklin, 17, shattered the world record in the 200-meter backstroke by three-quarters of a second. And Katie Ledecky, making her international debut at the tender age of 15 and given little hope of staying with the high-profile swimmers in the 800-meter field, won the longest race in women's Olympic swimming by more than 4 seconds.
The United States came close to a sweep of the four gold medals awarded Friday, but Florent Manaudou of France won the 50-meter freestyle by two-tenths of a second over American Cullen Jones. Manaudou's sister won the 400-meter freestyle at the Athens Olympics in 2004.
Phelps has had his highs and lows during the Olympics and one of the biggest lows of his career came when he lost by .05 of a second to South African Chad le Clos in the 200-meter butterfly.
Le Clos was on hand again, hoping to make it a double victory over Phelps, and there were others in the field with a chance as well.
It took Phelps 24.35 seconds to cover the first 50 meters and as he made his turn he was in seventh place. Over the final 50 meters, he looked like a different swimmer and as he neared the wall it was clear he had established a clear advantage -- at least as clear as one can hope to have in a 100-meter race.
Phelps' winning time of 51.21 was slower than the one he turned in during the semifinals, but it was 0.23 of a second faster than le Clos and Russia's Evgeny Korotyshkin, who finished in a dead heat for second place.
Franklin swam the 200-meter backstroke in 2:04.06, breaking the world mark of 2:04.81. Less than a quarter into the race it was obvious that everyone else was fighting for second place and that spot went to Russia's Anastasia Zueva. American Elizabeth Beisel finished third and won one of the five medals handed out to the United States in the pool Friday.
Ledecky's performance came as a huge surprise to those watching and to those swimming as well.
She sped away at the start on a pace that seemed much to fast to maintain. The other swimmers didn't try to challenge her, assuming she would soon fade over the long grind of 800 meters. That never happened and Ledecky finished in 8:14.63, an American record.
Mireia Belmonte Garcia won her second silver medal of the Olympics, but she was 4.13 behind Ledecky. Pre-race favorite Rebecca Adlington settled for the bronze.