US swimmer Michael Phelps swims fast, but not fast enough to beat Uncle Sam, who was awaiting him at the finish line each time he won a medal in Rio.

His total income tax bill for the 2016 Games? Up to about $US55,000 ($71,500) for his five golds and one silver.

US Olympic athletes who bring home medals also bring home cash - $25,000 for gold, $15,000 for silver and $10,000 for bronze - paid for by the United States Olympic Committee.

Like any prize winner, from a jackpot hitter to a Nobel Prize recipient, the athletes are taxed because Olympic medals and cash bonuses are considered income, said Steven Gill, associate professor of accounting at San Diego State University.