In a statement to The Associated Press, the swimmer who won a record eight gold medals at the Beijing Games did not dispute the authenticity of the exclusive picture published Sunday by the tabloid News of the World.
"I engaged in behavior which was regrettable and demonstrated bad judgment," Phelps said in the statement released by one of his agents. "I'm 23 years old and despite the successes I've had in the pool, I acted in a youthful and inappropriate way, not in a manner people have come to expect from me. For this, I am sorry. I promise my fans and the public it will not happen again."
News of the World said the picture was taken during a November house party while Phelps was visiting the University of South Carolina. During that trip, he attended one of the school's football games and received a big ovation when he was introduced to the crowd.
While the newspaper did not specifically allege that Phelps was smoking pot, it did say the pipe is generally used for that purpose and anonymously quoted a partygoer who said the Olympic champion was "out of control from the moment he got there."
The U.S. Olympic Committee said it was "disappointed in the behavior recently exhibited by Michael Phelps," who was selected the group's sportsman of the year. He also was honored as AP male athlete of the year, and his feat in Beijing - breaking Mark Spitz's 36-year-old record for most gold medals in an Olympics - was chosen as the top story of 2008.
"Michael is a role model, and he is well aware of the responsibilities and accountability that come with setting a positive example for others, particularly young people," the USOC said in a statement. "In this instance, regrettably, he failed to fulfill those responsibilities."
The party occurred nearly three months after the Olympics while Phelps was taking a long break from training, and this apparently would have no impact on the eight golds he won at Beijing. He has never tested positive for banned substances and even agreed to extra testing before the games.
Marijuana is viewed differently from performance-enhancing drugs, according to David Howman, executive director of the World Anti-Doping Agency. An athlete is subject to WADA sanctions only for a positive test that occurs during competition periods.
"We don't have any jurisdiction," Howman said. "It's not banned out of competition. It's only if you test positive in competition."
Phelps returned to the pool a couple of weeks ago to begin preparations for this summer's world championships in Rome. He plans to take part in his first post-Olympics meet in early March, a Grand Prix event in Austin, Texas.
Phelps was in Tampa, Fla., during Super Bowl week to make promotional appearances on behalf of a sponsor. But he left the city before Sunday's game between the Pittsburgh Steelers and Arizona Cardinals, abandoning his original plan to be at Raymond James Stadium.
This isn't the first embarrassing episode for Phelps after an Olympic triumph. In 2004, a few months removed from winning six gold and two bronze medals in Athens, the swimmer was arrested on a drunken driving charge at age 19. He pleaded guilty and apologized for the mistake.
In his book "No Limits: The Will to Succeed," Phelps recounted how his first phone call was to his agent, and not his mother or coach Bob Bowman, because he knew they would yell at him.
Later, he called Bowman, who was supportive but told him, "Michael, just because you want to blow off some steam doesn't mean you can be an idiot."
Debbie Phelps, his mother, cried at the news.
"That hurt worse, maybe, than anything," Phelps wrote. "I had never seen my mother that upset."
Bowman did not immediately respond to phone and e-mail messages Sunday.
Last year, News of the World posted video on its Web site showing Max Mosley, the president of motor racing's governing body, engaging in sex acts with five prostitutes. The video was filmed by one of the women, and Mosley admitted to being a part of the scenario but sued for breach of privacy and was awarded $120,000 in damages. Another news break involved Prince Harry in 2002, smoking marijuana and drinking before the legal age of 18.
Olympic teammate Dara Torres said Phelps has become such a prominent figure that everything he does is news.
However, she said: "This in no way, shape or form diminishes anything he's done."
"It's sort of a double-edged sword," Torres told the AP on Sunday. "When you're recognizable, you're looked up to as a role model. He is recognizable and everything you do gets looked at and picked apart. I guess that's the price of winning 14 Olympic medals."
Jason Lezak, whose remarkable anchor leg of the 400-meter freestyle relay helped Phelps stay on course to break Spitz's record, said he was "saddened" to hear of the report.
"While I don't condone his conduct, I am a teammate and fan," Lezak said in a text message to the AP. "Unlike many fair-weather people, I am sticking by him. If my wife and I can help him in any way, we will. I believe he will grow from this and be better person, role model and teammate."
The USOC noted that Phelps acknowledged his mistake and apologized.
"We are confident that, going forward, Michael will consistently set the type of example we all expect from a great Olympic champion," the governing body said.
By AP National Writer Paul Newberry; AP Sports Writers Beth Harris in Los Angeles and Chris Lehourites in London contributed to this report