Team USA's Nathan Adrian Diagnosed with Cancer, Gunning For 2020 Olympics

Team USA swimmer Nathan Adrian has announced he's battling testicular cancer ... but the gold medalist says the diagnosis will not derail his plans to compete at the 2020 Olympics.

The 30-year-old -- who was part of the 4x100 relay that won gold with Michael Phelps in 2008 -- made the announcement on social media ... saying he initially went to the doctor after "something didn't feel right."

After several tests, doctors discovered the disease and began treatment shortly after. Luckily for Adrian, he says the cancer was caught early and the prognosis is good ... which means he'll still be able to train for the Tokyo Olympics.

"I will be back in the water in a few short weeks with my sights fully set on Tokyo!" Adrian says.

"As I told my family, I’ll be putting my public health degree to work a little sooner than I planned! But in all seriousness, I am keeping a positive attitude as cases such as mine are curable."

Adrian says he will have surgery next week ... and will share his journey to bring light to men's health issues.