"Life, like swimming the 100 free, can come at you hard and fast as you can't always see who, or what, may be chasing you down," said the American, who won two relay golds in Rio de Janeiro 2016 in a U.S. team that included Michael Phelps.
Adrian also won two golds at London 2012, one of them the 100m freestyle, and another at Beijing 2008.
"Recently I went to the doctor as something didn't seem quite right...after a few tests and visits with a specialist, I unfortunately learned that I have testicular cancer," he said.
"On the bright side, we caught it early, I have already started treatment and the prognosis is good. I will be back in the water in a few short weeks with my sights fully set on Tokyo," he added.
Adrian also has an Olympic silver and two bronzes, including the 50 and 100m freestyle at Rio.
He has eight long course world championship gold medals, three silvers and two bronzes.
(Reporting by Alan Baldwin in London, editing by Toby Davis)
