Michael Phelps, the most successful Olympian of all time, will race against a great white shark as part of a television show due to air in July.
A press release from the Discovery Channel promises that "the world's most decorated athlete takes on the ocean's most efficient predator".
Phelps, 31, has 23 Olympic swimming gold medals and retired after Rio 2016.
The American has a top speed of six miles per hour in the water, whereas a great white can reach 25mph.
"Phelps v Shark: Great Gold v Great White" will form part of the channel's "Shark Week" which begins on 23 July.
What form the race will take "is anyone's guess" according to Tooni Mahto from the Australian Marine Conservation Society.
She told BBC Radio 5 live "it is a forgone conclusion as to who is going to come out on top".
But she added: "We can all be fairly certain it's not going to be Michael Phelps versus a great white shark in an Olympic-size swimming pool.
"I'm assuming it's going to be looking at the human body in water compared to a creature that's evolved over millions of years and is perfectly adapted to its marine environment."