The 18-time Olympic champion had been scheduled for trial on Nov. 19. It's been moved to Dec. 19 at the request of his lawyer.
Phelps was charged Sept. 30 with DUI, excessive speed and crossing double lane lines on Interstate 95 in his native Baltimore. Police say he registered .14 per cent on a blood-alcohol test; The legal limit is .08 per cent in Maryland.
A week after that, USA Swimming suspended the 29-year-old for six months and forced him to withdraw from next year's world championships.
If convicted, Phelps faces up to a year in jail.
In 2004, Phelps pleaded guilty to DUI on Maryland's Eastern Shore. His conviction was waived because he was a young first-time offender.