"I'm honoured to be chosen, proud to represent the US, and humbled by the significance of carrying the flag and all it stands for," Phelps said on Wednesday.
"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," added the the 31-year-old, who has won 22 Olympic medals, 18 of them gold, over four Games
"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."
In the past Phelps would have competed in the 400 metres individual medley, which takes place on the first day of competition in the pool, preventing him from being considered as flagbearer, reports Xinhua.
However, he will not contest the event in Rio, freeing him up to become only the second swimmer in history to carry the US flag after Gary Hall at the Montreal 1976 Olympic Games.
Phelps will compete in 100m and 200m fly and 200m medley at the Rio Games.
The Opening Ceremony will take place on August 5 at the Maracana stadium.
Phelps had retired from the sport after the London Games but decided to come back 18 months later.