He will be the first swimmer to be the flag-bearer since Gary Hall in 1976, leading the American team in Friday's opening ceremony.
"I'm honored to be chosen, proud to represent the U.S., and humbled by the significance of carrying the flag and all it stands for," said Phelps, who is the most decorated Olympian of all time with 22 medals.
"For Sydney, I just wanted to make the team. For Athens, I wanted to win gold for my country. For Beijing, I wanted to do something nobody else had done. In London, I wanted to make history.
"And now, I want to walk in the opening ceremony, take it all in, represent America in the best possible way and make my family proud. This time around, it's about so much more than medals."
Phelps retired after the London 2012 Olympics, where he took his career haul to 18 gold medals, but came out of retirement two years later.
He was banned for six months after being arrested for drink driving and speeding, missing the 2015 world championships, but subsequently qualified for the 100 and 200 meters butterfly plus the 200m individual medley.
