The younger son of Prince Charles and Diana, princess of Wales, was at a huge after-hours pool party at the Wynn Las Vegas resort and casino when he and Lochte -- who holds the world record in the 200-meter and 400-meter individual medleys -- decided to race at the hotel's extravagant XS nightclub pool, the New York Daily News reported, citing video it received.
They agreed to do the breaststroke.
Young women in bikinis watched as Lochte counted to three on his left hand, then blasted to a quick lead over the 27-year-old prince, the News said.
The Olympic icon and Swimming World Magazine 2011 World Swimmer of the Year easily won the race, raising both arms in victory and hugging Harry amid 5,000 fellow partiers.
Harry, officially Prince Henry of Wales, had a similar faceoff with Jamaican sprinter Usain Bolt, widely regarded as the fastest person ever -- but with a different result.
The two were at a track at the University of the West Indies in March. When the race started, Harry sprinted as fast as he could, while the gold medalist jogged after him laughing, the News said.
Prince Harry was in Las Vegas after hosting a charity event in San Diego, Buckingham Palace said.