Marketing experts said the 23-year-old American, who is now the most successful Olympian with 11 gold medals, will become the richest professional swimmer ever, far surpassing the money earned by the former most decorated U.S. swimmer, Mark Spitz.

"He's the greatest Olympian in the world and he'll be able to earn money everywhere as he's an international brand," Australia-based celebrity agent Max Markson told Reuters.

"He's a billion dollar man. He won't have to get a job ever. He can live off this for 50 years."

Olympic sports have meant big business since the Olympic movement allowed professional athletes to compete 20 years ago.

But none has banked the sums earned by charismatic megastars like Tiger Woods, David Beckham or Michael Jordan whose names are globally known and set cash registers ringing everywhere.