Katie Ledecky just swam the entire length of a pool while balancing a glass of milk on her head ... and she didn't spill a single drop the whole time!!!

The 5-time Olympic gold medalist posted the insane swim on her social media page as part of an ad for milk ... and the footage is straight-up mind-blowing.

Ledecky, sporting snorkel gear, hopped into the pool and put the glass of chocolate goodness right on the back of her head.

What happened next was amazing ... she cruised through the pool no problem, with not one ounce of milk wasted!!!

In fact, to prove it all was real, after she got up for air -- she took the glass off her head and chugged down a sip!!!

"Possibly one of the best swims of my career! (~open for debate~)," Ledecky said.