Phelps, 29, was stopped for speeding just outside the Fort McHenry Tunnel about 1:40 a.m. Tuesday, Maryland Transportation Authority Police said. He was clocked going 84 mph in a 45-mph zone in his 2014 Land Rover and subsequently failed a standard field sobriety test, police said.
Phelps posted an apology on social media Tuesday afternoon, saying: "I understand the severity of my actions and take full responsibility. I know these words may not mean much right now but I am deeply sorry to everyone I have let down."
Police, who said he was cooperative, also said Phelps failed a Breathalyzer test but would not confirm the results.
The swimmer was charged with DUI, excessive speed, and crossing double lane lines. Police said he was not taken to Central Booking but processed at a transportation authority command station and released to a "responsible adult." His vehicle remained on the shoulder of the road for hours Tuesday before being picked up in the afternoon. The DUI charge carries a possible penalty of a year in jail, though one legal expert said it was unlikely Phelps would get jail time.
The arrest of the decorated Olympic swimmer from Rodgers Forge was another low in a month of bleak off-field stories for Baltimore sports. Earlier in September, the Ravens released star running back Ray Rice after TMZ posted video of him felling his then-fiancee with a punch. The same week, popular Orioles slugger Chris Davis was suspended 25 games for amphetamine use as his team was about to clinch its first American League East title since 1997.
Phelps' agent, Drew Johnson, offered no comment beyond Phelps' Twitter apology. His longtime coach, Bob Bowman, did not respond to a request for comment.
Phelps pleaded guilty to driving while impaired after a 2004 arrest in Wicomico County, when he was 19. He encountered further controversy in 2009, when British tabloids published photos of him smoking a marijuana pipe at a college party.
In both cases, Phelps apologized and promised better judgment going forward. Given his past difficulties, the arrest raised questions about Phelps' dedication to his 5-month-old comeback and about his appeal to sponsors as the leading figure in his sport. Though Phelps hasn't formally committed to swimming at the 2016 Olympics in Rio de Janeiro, he has promised to train more vigorously to improve his results against elite competition.
"To be quite honest, it's like a punch in the stomach," said NBC commentator and former Olympic gold medalist Rowdy Gaines. "I felt like I was going to throw up. I know that sounds a little dramatic, but he's the face of our sport."
Phelps was suspended three months by USA Swimming in 2009 for the photo of him smoking a marijuana pipe. Asked if he might face suspension for this arrest, a spokesman for the governing body did not respond immediately.
Phelps retained his commercial appeal and competitive form through his previous missteps. He has sponsorship deals with Under Armour, Subway and Aqua Sphere, a company for which he and Bowman are creating a new line of swim gear. Phelps has also been a spokesman for brands such as Visa, OMEGA watches, Louis Vuitton luggage and the Hilton hotel chain, among others.
It wasn't immediately apparent how his arrest might affect his existing deals; spokespersons for Under Armour, Subway, Aqua Sphere and OMEGA did not respond to requests for comment Tuesday.
But Phelps will likely remain a popular corporate pitchman, said Bob Dorfman, executive creative director for San Francisco-based Baker Street Advertising.
"A second DUI is going to make sponsors nervous," Dorfman said. "Fortunately for Phelps, the 2016 Games are still a ways off, and there's plenty of time for this to be forgotten before the serious Rio marketing push begins. … As long as he stays squeaky clean and swims well from now to Rio, I don't see this affecting his marketability. His comeback story is simply too compelling to consumers and marketers."
He said Phelps might actually be helped by the negative headlines swirling around other athletes, including Rice, because his offense might be lost in the shuffle.
Matt Delzell, managing director of Dallas-based The Marketing Arm, said some sponsors might "drop him while some will stand by him, but have him on a tighter leash."
He said Phelps could face particular wariness from more family-oriented sponsors such as Procter & Gamble, which signed the swimmer to a seven-figure deal leading to the 2012 Olympics.
"This is not good, obviously, given the fact that it's his third substance-abuse issue," Delzell said. "For a guy who said, 'I won't make the same mistake twice,' at some point, you're the boy who cried wolf."
Mothers Against Drunk Driving said Phelps should have learned from his first arrest and called for Phelps, if convicted, to be required to install an ignition interlock device, which prevents drunken driving.
"A first instance of drunk driving is unacceptable, let alone repeat occurrences," MADD said.
Lisa Spicknall, state director of MADD, said Phelps spoke to a MADD panel as part of court-ordered probation after his 2004 plea but had not been a participant in recent efforts.
"He's a high-profile person in Maryland, and a lot of kids look up to him," she said.
She said statistics show people drive impaired 87 times before they are caught by police. "And he's been caught twice," she said.
Maryland law gives prosecutors the power to seek enhanced maximum penalties for second-time driving-under-the-influence or driving-while-intoxicated offenders. The Baltimore state's attorney's office declined to comment on the case.
But Baltimore County State's Attorney Scott D. Shellenberger, who is not handling the case, said because Phelps received a probation before judgment in 2004 — an outcome that let him avoid a conviction — prosecutors cannot seek the enhanced penalties.
Kush Arora, a Maryland-based attorney who regularly handles drunken-driving cases, said it would be highly unusual for Phelps to receive any jail time if he is found guilty in the case, given what has been made public so far.
"I would have a really hard time believing judges in most of our counties in this state would believe that a period of incarceration was appropriate," he said.
Around the swimming world, friends and associates reacted to Phelps' arrest.
His longtime friend and rival, Ryan Lochte, told TMZ: "I'm not concerned. ... He just makes dumb decisions. … He has so much money to get a driver. I even have a driver. Just stinks for the sport of swimming."
Gaines said one thing he's not worried about is Phelps' devotion to his comeback. "This has nothing to do with that," he said. "September is a vacation month in the sport, the time to lay low."
Phelps retired after the 2012 Olympics in London, where he pushed his record totals to 18 gold medals and 22 medals overall.
In the months that followed, he traveled, played golf and high-stakes poker and generally enjoyed a life of leisure. But Phelps resumed training late last year, at first casually and then more seriously as he contemplated a return to competition.
He resumed racing at the Mesa (Ariz.) Grand Prix in April, sporting a grin as he talked about how happy he was to swim again, this time on his own terms.
The sport was thrilled to have him as well, with Phelps attracting sellout crowds and national media coverage to races that seldom receive such attention in non-Olympic years.
Though Phelps has performed unevenly, he posted the fastest 100-meter butterfly time of 2014 at the Phillips 66 National Championships in August. His year climaxed later that month at the Pan Pacific championships in Australia, where he won three gold medals and two silvers.
He's up for five awards at USA Swimming's coming Golden Goggles ceremony in November, including Male Athlete of the Year and the Perseverance Award.
His next major international meet would be world championships next summer in Russia.
Baltimore Sun reporters Ian Duncan and Jean Marbella contributed to this article.