Olympic gold medal-winning swimmer Michael Phelps has pleaded guilty to drink-driving.

Phelps entered his plea to a charge of driving under the influence on Friday. He was sentenced to a year and a half of probation.

The 18-time gold medallist was arrested on September 30.

Documents show that Phelps, 29, was leaving the Horseshoe Casino in central Baltimore when he was pulled over.

Police say Phelps registered a 0.14 per cent on a blood-alcohol test. The legal limit to drive in Maryland is 0.08 per cent.

A week after his arrest, USA Swimming suspended Phelps for six months.

Phelps entered a six-week treatment programme in October.

This is his second DUI conviction. Phelps pleaded guilty to drink-driving in 2004 in another Maryland county.

"I now have the tools to move past this. What I did was wrong, and I made a bad mistake. I'm looking forward to having a much brighter future than I had in the past," Phelps told the judge.

Judge Nathan Braverman told Phelps that success overcoming alcohol would not come overnight, and warned him of the consequences of another slip-up.

"You don't need a lecture from the court," Judge Braverman said. "If you haven't gotten the message by now, or forget the message, the only option is jail."