The 29-year-old swimmer is the most decorated Olympian of all-time.
The 18-time Olympic gold medalist was also sentenced to 18 months of supervised probation.
During that time Michael Phelps must totally abstain from drinking alcohol.
Michael Phelps tested at nearly twice the legal alcohol limit when he was arrested in Baltimore in September.
He was initially stopped for speeding after being caught going at 84mph in a 45mph zone.
The swimmer said he had also spent time at the Meadows addiction treatment centre in Arizona.
“During my 45-day program, I was able to find out a lot about myself that I never knew,” Michael Phelps told reporters.
“I’m looking at a much brighter future than I have had in the past,” he added.
Michael Phelps retired after winning his 22nd Olympic medal at the London Games in 2012 but came out of retirement earlier this year.
He was given a six-month ban from swimming after his arrest in September, but continues to train.
Michael Phelps, who is from Baltimore County, was previously charged for DUI in Maryland in 2004. He pleaded guilty to driving while impaired in exchange for 18 months’ probation.
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