Michael Phelps has been banned from competing for three months after photos were printed which appeared to show him smoking an illegal drug.
The US swimmer was called the Greatest Olympian of all time when he won eight gold medals in Beijing last summer.
Phelps hasn't said whether or not he took cannabis, but said he does regret some of his behaviour in the past.
He said, "For this, I am sorry. I promise my fans and the public it will not happen again."
The photo was published by a newspaper in Britain, and was thought to have been taken at a party last November, when Phelps was on a break from training.
Phelps won eight golds at the 2008 Beijing Olympics
The people in charge of swimming in America said the 23-year-old hadn't broken any drug rules in swimming competitions.
But officials said the photo had disappointed lots of people - especially the thousands of kids that look up to him.
A statement from USA Swimming said: "We decided to send a strong message to Michael because he disappointed so many people, particularly the thousands of USA Swimming member kids who look up to him as a role model and a hero."