On Sunday, he received a warm welcome from the estimated 40,000 people gathered in front of Buckingham Palace for the Visa London 2012 party that took place as the Beijing Olympics came to an end.
In an interview onstage, Phelps, 23, summed up his experience in Beijing, where he took home a record-breaking eight gold medals: "It was the perfect week, the perfect races – couldn't ask for anything else."
Along with Phelps's appearance and free concerts throughout the day, the London crowd watched the Beijing 2008 Olympics closing ceremony in which David Beckham, Jimmy Page and Leona Lewis contributed to the handover segment.
Before Phelps left the stage, the Red Arrows team of nine jets performed a fly-by overhead, leaving trails of red, white and blue smoke above Buckingham Palace.
"Too cool. I've never seen something like that before," he said.
When asked what the Games might be like in 2012, he said, "Hard to imagine, but this is a great start. It's going to be amazing. I'm kind of at a loss for words because I was like, 'Oh, you know, I wonder how many people are going to be there.' I'm staring down this street of people just packed together. It's one of the coolest things I've ever experienced."
And this from a man who just set seven world records.