The U.S. Olympic Committee said on Wednesday that Phelps, who will be the first American male swimmer to compete at five Games, had been chosen in a vote of fellow team members to lead the delegation into the Maracana stadium.
Phelps, who won eight of his golds in Beijing in 2008, is set to add to his haul in Rio with three individual events as well as relays.
"I'm honoured 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.
"For Sydney (in 2000), 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.
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"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."
Taking part in the opening ceremony will mean a late night for Phelps on Friday, with the stadium at least an hour away from the Athletes Village in Barra, and plenty of standing around as well.
The 31-year-old will not be in action on the opening Saturday, however, with his first event likely to be the men's 4x100 freestyle relay on Sunday.
The U.S. team have not announced their lineup for that event and Phelps is unlikely to take part in the afternoon heats, meaning he will make his competitive entry in the pool late in the evening if selected.
(Reporting by Alan Baldwin, editing by Toby Davis)
