American swimmer Emma Weber had lost her cap in the fourth heat on Sunday and before the fifth heat got underway, a lifeguard was called upon to retrieve the cap from the bottom of the pool.

At first, the confused crowd in the 15,000-seat arena did not understand what was going on but when the lifeguard dove into the water and retrieved the white cap, they quickly realised what was happening and gave him a loud cheer.

On his part, the lifeguard seized his Olympic moment and as he climbed out of the pool, he gave the fans a quick wave before adjusting his trunks and walking off.

“This situation is common at swimming competitions,” a Games spokesman was quoted as saying by CNN.

“As the lifeguards are the only people wearing swimwear, apart from the athletes, a request may be made to fetch an item at the bottom of the pool prior to the next race.

“In this case, the request was made to the lifeguard closest to the item.”

The spokesman added that the lifeguard did not want to be named but American broadcaster NBC gave him the moniker “Bob the cap catcher” and it has stuck ever since he went viral on social media.