The arrest last Tuesday marked the second time the 29-year-old Baltimore-area native had been arrested for drunken driving, the first being in 2004. In the first case, he pleaded guilty to a lesser charge of driving while impaired in exchange for 18 months' probation.
"Membership in USA Swimming, and particularly at the National Team level, includes a clear obligation to adhere to our Code of Conduct," USA Swimming Executive Director Chuck Wielgus said in a statement.
"Should an infraction occur, it is our responsibility to take appropriate action based on the individual case. Michael's conduct was serious and required significant consequences."
Phelps was arrested last week after speeding and then crossing the double-lane lines inside a Baltimore tunnel, police said, adding he was clocked by radar traveling 84 miles per hour (135 kph) in a 45-mph (72-kph) zone.
Documents indicate that Phelps registered .14 percent on a Breathalyzer test after being pulled over in his 2014 Land Rover. The legal limit for intoxication in Maryland is .08.
Phelps apologized for the incident and said on Sunday he was checking himself into rehab for six weeks in order to "better understand myself."
He can train with his member club during his suspension, but is ineligible to participate in USA Swimming-sanctioned competitions through March 6, 2015. Phelps, who has won 22 Olympic medals, 18 of them gold, will also forfeit a monthly stipend from USA Swimming during the six-month suspension.
"Michael accepts USA Swimming's sanctions," said a spokesman at Octagon, the group that represents Phelps. "He has apologized for his actions and, as he shared yesterday, is taking steps to address them."
Although he has not indicated a desire to compete at the 2016 Olympics, many view Phelps' recent return to the pool as a clear sign he wants to stage a comeback.
Dara Torres, a 12-time Olympic medalist, said she believed Phelps could still make the U.S. squad for Rio de Janeiro.
"I don't think the suspension is going to affect him physically, because they're not saying he can't train. He just can't compete," Torres, a former teammate of Phelps, told Reuters in a telephone interview.
"He can still stay in shape and he knows what to expect and how to compete. It's not like he's starting something new."
Wielgus said: "Michael has publicly acknowledged the impact of his decisions, his accountability especially due to his stature in the sport and the steps necessary for self-improvement. We endorse and are here to fully support his personal development actions." (Reporting by Steve Ginsburg in Washington; Editing by Peter Cooney)