After winning a record-breaking eight gold medals at the Beijing Games, US swimmer Phelps will appear on Kellogg's Frosted Flakes and Kellogg's Corn Flakes cereal boxes from mid-September.
However, some nutritionists have hit out at the move, pointing out that Frosted Flakes (the US equivalent of Frosties) contains three times as much sugar as Wheaties and a third of the fibre. Wheaties traditionally features Olympians on its boxes.
They have argued that Phelps, who already has a sponsorship agreement with McDonald's, is encouraging already overweight American children to eat more.
Douglas Castle, senior advisor to the Children's International Obesity Foundation (CIOF), said: "Public figures like Michael Phelps exert a major influence over our youngsters.
"CIOF believes that celebrities should think twice before choosing to endorse or encourage the consumption of any product which is inherently unhealthful (sic) to children, especially if that product is correlated to obesity, diabetes and a myriad of dangerous conditions."
Marta Cyhan, vice-president of global promotions at the Kellogg Company, said: "Michael embodies the values behind our Frosted Flakes Earn Your Stripes programme.
"As an official sponsor of the 2008 US Olympic team and a proud sponsor of Phelps, it is only fitting that Kellogg Company feature this world-class athlete on its iconic boxes of Kellogg's Corn Flakes and Kellogg's Frosted Flakes cereals."
Phelps is currently appearing in a Visa commercial which celebrates his record-breaking achievement.
The TBWA/Chiat/Day ad is part of Visa's 'Go World' campaign. It features footage showing Phelps' part in the US swimming team's 4 x 100 freestyle relay victory.