The 25-year-old Rodgers Forge native, the most decorated Olympian in history, with 14 gold medals in swimming, has teamed up with an Indiana company to design and market a new line of mini-pools, in which users swim against a current of water.
Master Spas, of Fort Wayne, said Phelps and his coach, Bob Bowman, worked with the company's engineers to design six swim spas for home use as well as professional spas for fitness centers, universities and physical therapy centers.
Terms of the endorsement deal were not disclosed. Phelps could not be reached for comment.
The endorsement is the second this year for Phelps, whose deal with Baltimore-based Under Armour was announced in February.
Marketing experts had estimated that Phelps could earn $100 million from endorsements in his lifetime. But his prospects were dealt a blow early last year, when a photo emerged on the Internet of the swimmer appearing to smoke marijuana.
He lost a contract with Kellogg, but other sponsors, such as Subway, Speedo and Visa, stuck by him. Since then, Phelps has slowly worked his way back to getting endorsements.
In February, he signed the Under Armour deal, in which he agreed to wear the company's athletic apparel outside the pool and appear in commercials.
In June 2009, Phelps signed a multiyear deal with H2O Audio, a San Diego-based company that produces waterproof headphones and cases used by divers.
In January last year, before the marijuana photo surfaced in early February, Phelps signed a $1 million endorsement deal to promote Mazda in China.
This latest endorsement with Master Spas involves promotion of a niche product: specialized, high-performance pools that shoot streams of water and allow swimmers to practice their strokes.
The new spa line is known as the Michael Phelps Signature Swim Spas.
On Wednesday, Bob Lauter, chief executive of Master Spas, was at the International Pool Spas Patio Expo in Las Vegas, where he introduced the new line with Phelps in attendance. The spa pools start at $25,000 and go up to about $45,000 and will be available for sale later this month, Lauter said.
Lauter, who said his company sells swim spas all over the world, said he was excited that Phelps was affiliated with Master Spas because the Olympian has instant name recognition around the world.
"He's a global icon," Lauter said of Phelps. "Every market we're in, they know who he is."
Lauter said that Phelps' 2009 brush with bad publicity didn't faze him. "I didn't even think twice about that before we went forward," Lauter said.