"Possibly one of the best swims of my career! Check out the full swim here. What can you do without spilling a drop?!" Ledecky wrote in her caption, adding the “#GotMilkChallenge” hashtag.
“I swam for three years in college so I feel like I am qualified to say that Katie Ledecky is not a human but a swimming robot,” one Twitter user wrote.
“Sometimes you think you’ve seen it all in sports, and then you come across what might be the most athletic thing I’ve ever seen,” another user wrote.
In her Instagram story, Ledecky also challenged tennis player Sloane Stephens to complete the challenge.
Stephens on Monday shared a video of herself hitting tennis balls skillfully while holding a glass of milk.
The “Got Milk?” ad campaign was first launched by advertising agency Goodby Silverstein & Partners for the California Milk Processor Board in 1993.
The purpose of the campaign was to encourage people to drink milk, and it heavily relied on print ads of celebrities, including Britney Spears, Beyoncé and Serena Williams, sporting “milk mustaches.”