Phelps won a 20th Olympic gold — and 12th individual title — with a dominant victory in his signature event, the 200m butterfly, before anchoring the US to gold in the 4x200m freestyle relay.
A disbelieving Phelps was still absorbing his achievement as the most decorated Olympian in history, with 25 medals in all, when he targeted further success in his fifth and final Games.
“That’s a lot of medals,” he said. “It’s just insane. It’s mind-blowing.
“I wanted that one back. I came into the pool with a mission and the mission was accomplished.”
He has three more opportunities in Rio, his final Olympics, in the 100m butterfly, 4x100m medley relay, and 200m individual medley.
The “mission” was a cathartic victory in the event in which he made his name, finishing fifth in the Sydney Olympics 16 years ago, aged 15.
South African Chad le Clos had shocked Phelps, the 2004 and 2008 Olympic champion, by winning gold ahead of his idol at the London 2012 Games.
Phelps retired after those Games, as planned, but he returned 18 months later with unfinished business.