Commissioner Roger Goodell handed down the punishment a week after prosecutors decided not to charge Roethlisberger in a case involving a 20-year-old college student who accused him of sexually assaulting her in a Georgia nightclub in March.
Roethlisberger also was ordered to undergo a comprehensive behavioral evaluation. Goodell will look at Roethlisberger's progress before the season and might consider reducing the suspension to four games.
However, a failure to comply with the NFL's ruling might lead to a longer suspension.
Roethlisberger is barred from attending any Steelers on-field, offseason activity until he completes the evaluation. He can participate in training camp and preseason games this summer as long as he is cleared by the evaluators and Goodell.
Roethlisberger is the first player suspended by Goodell under the conduct policy who hasn't been arrested or charged with a crime.
"He was disappointed by the number of games [of the suspension], as was I, frankly," Roethlisberger's attorney, David Cornwell, said Thursday on ESPN Radio's "Mike & Mike in the Morning."
Steelers president Art Rooney II said Wednesday, "I agree and support the decision the commissioner made today."
The Steelers have begun contacting teams about trading a top-10 pick for Roethlisberger, an NFL source told ESPN NFL Insider Adam Schefter. The Steelers have called the Rams, 49ers, Raiders, Bills, Jaguars, Seahawks and Browns, the source said.
The Rams and Bills are known to be not interested, the source said.
Rooney didn't deny that the team was shopping Roethlisberger, but didn't verify the report, either, when he answered a question during Wednesday's media conference call.
"We really can't answer questions about trades, particularly [right before the NFL draft]," Rooney said.
What the Steelers can't quantify is what effect Roethlisberger's absence will have on a team one season removed from winning its second Super Bowl in four years.
"You're going to have stumbling blocks," wide receiver Antwaan Randle El said. "You've got to be able to stay together and move through it, because if you don't, you'll have the problems later on."
There are plenty of championship pieces remaining from that team, including 18 starters, for the Steelers to contend again. The schedule appears to be accommodating. The Steelers play three teams that didn't make the playoffs this past season -- the Falcons, Buccaneers and Tennessee Titans -- plus the division rival Baltimore Ravens in their first four games.
Pittsburgh hosts the Cleveland Browns in its fifth game and travels to Miami to face the Dolphins in its sixth game. The Steelers have a bye in Week 5, meaning Roethlisberger might have two weeks to practice before he plays, if his suspension is reduced.
There's nothing to suggest NFL schedule makers were aware of the length of Roethlisberger's suspension in advance. Still, the first of the Steelers' five prime-time games isn't until Oct. 31 -- or after his potential return.
Some teammates don't doubt he will be back, either, despite the enormity of the suspension. Or the animosity that exists among a Steelers fan base that is disgusted with his behavior -- to some, a quarterback sack now means throwing Roethlisberger's No. 7 jersey into a trash can.
"I think if Ben has shown anything, it's that he's able to recover whenever he has been faced with adversity," safety Troy Polamalu said. "Everything is not always peachy. Not everybody has that unblemished image. If he has that repentance, the way he is going to have to have it to continue to live his life, this will be the great example of that."
Sitting out all six games would cost Roethlisberger an estimated $2.8 million of his $102 million total deal, though the penalty could be shortened to four games for good behavior.
The Steelers also were ordered to remit a portion of Roethlisberger's salary to the NFL for having two players suspended for violations of the league's personal conduct or drug policies. Wide receiver Santonio Holmes was suspended for four games before being traded to the New York Jets this month.
A second player being suspended requires the offending team to pay the NFL 25 percent of the second player's forfeited salary, with the maximum amount being $200,000.
Goodell said before acting, he interviewed Roethlisberger on April 13, and talked to current and former players and the players' union. He also reviewed information from the Georgia Bureau of Investigation and Milledgeville police, and talked privately with Georgia district attorney Fred Bright.
In addition, Goodell said he listened to recommendations from the quarterback's representatives and took into account information learned by the NFL office regarding the alleged assault.
"The Personal Conduct Policy makes clear that I may impose discipline 'even where the conduct does not result in conviction of a crime' as, for example, where the conduct 'imposes inherent danger to the safety and well being of another person,' " Goodell stated in his letter to Roethlisberger.
"As the District Attorney concluded, the extensive investigatory record shows that you contributed to the irresponsible consumption of alcohol by purchasing [or facilitating the purchase of] alcoholic beverages for underage college students, at least some of whom were likely already intoxicated. There is no question that the excessive consumption of alcohol that evening put the students and yourself at risk. The Personal Conduct Policy also states that discipline is appropriate for conduct that 'undermines or puts at risk the integrity and reputation of the NFL, NFL clubs, or NFL players.' By any measure, your conduct satisfies that standard."
More excerpts from Goodell's letter to the Steelers quarterback:
• "I recognize that the allegations in Georgia were disputed and that they did not result in criminal charges being filed against you. My decision today is not based on a finding that you violated Georgia law, or on a conclusion that differs from that of the local prosecutor. That said, you are held to a higher standard as an NFL player, and there is nothing about your conduct in Milledgeville that can remotely be described as admirable, responsible, or consistent with either the values of the league or the expectations of our fans."
• "Your conduct raises sufficient concerns that I believe effective intervention now is the best step for your personal and professional welfare."
• "I believe it is essential that you take full advantage of the resources available to you. My ultimate disposition in this matter will be influenced by the extent to which you do so, what you learn as a result, and a demonstrated commitment to making positive change in your life."
• "In your six years in the NFL, you have first thrilled and now disappointed a great many people. I urge you to take full advantage of this opportunity to get your life and career back on track."
Goodell has aggressively dealt with players who violated the personal conduct policy throughout his 3½ years as commissioner. He banned the Titans' Adam "Pacman" Jones for one year, and suspended Chicago's Tank Johnson and Cincinnati's Chris Henry eight games each in 2007. Henry died last year of a head injury after falling off the back of a truck.
Last year, Michael Vick was suspended for six games, later shortened to two games, after serving 18 months in jail for his role in a dogfighting ring.
In all, 16 players have been suspended under the conduct policy by Goodell.
On Tuesday, the Steelers traded a seventh-round pick in this week's NFL draft to the Tampa Bay Buccaneers for quarterback Byron Leftwich, adding depth to the position in anticipation of the Roethlisberger suspension.
Leftwich's contract has been extended by one year through 2011, a source told ESPN's Chris Mortensen. He will earn $1.8 million in base salary this season and $2 million in 2011.
Leftwich backed up Roethlisberger in 2008, when the Steelers won the Super Bowl. He knows offensive coordinator Bruce Arians' system and could be ready to play Sept. 12 when the Steelers open against the Atlanta Falcons.
Pittsburgh already had third-year quarterback Dennis Dixon and longtime backup Charlie Batch under contract. Batch has been injured each of the past two seasons. Dixon has started only one NFL game but likely will compete with Leftwich in training camp to be the early-season starter.
In an e-mail to ESPN, David F. Walbert, the accuser's attorney, had no comment on the Roethlisberger suspension.
The NFL and the Steelers were angered and embarrassed by the tawdry details of Roethlisberger's night out March 3.
In a statement to police, the 20-year-old college student said Roethlisberger encouraged her and her friends to take numerous shots of alcohol. Then one of his bodyguards escorted her into a hallway at the Capital City nightclub in Milledgeville, Ga., sat her on a stool and left. She said Roethlisberger walked down the hallway and exposed himself.
"I told him it wasn't OK, no, we don't need to do this and I proceeded to get up and try to leave," she said. "I went to the first door I saw, which happened to be a bathroom."
According to her statement, Roethlisberger then followed her into the bathroom and shut the door.
"I still said no, this is not OK, and he then had sex with me," she wrote.
Two of her friends said they saw a bodyguard lead her into the hallway and then saw Roethlisberger follow. They said they couldn't see their friend but knew she was drunk and were worried about her.
Ann Marie Lubatti told police that she told the bodyguard, "This isn't right. My friend is back there with Ben. She needs to come back right now."
Lubatti said the man -- whom Georgia investigators later identified as Pennsylvania state trooper Ed Joyner -- wouldn't look her in the eye and said he didn't know what she was talking about.
Milledgeville is a central Georgia college town about 30 miles from where the quarterback owns a lake home.
The statements were among hundreds of pages of the case file made public last week by the Georgia Bureau of Investigation.
Last week, Steelers president Rooney said the team would have imposed sanctions on Roethlisberger before, but doing so would have permitted the players' union to appeal any penalties. The suspension imposed by Goodell can be appealed only to him.
It was also announced on Wednesday Joyner can no longer work as a personal assistant to Roethlisberger because of the Georgia incident.
The Pennsylvania State Police said Joyner's outside work exceeded the scope of what was permitted. They also say "he is alleged to have demeaned the image" of the state police.
Joyner got permission from the state police in 2005 to work as Roethlisberger's assistant. State police regulations require troopers to get permission for any outside work for which they'll receive compensation or "consideration."
Joyner did not immediately return a call seeking comment at his barracks in Washington, Pa., where he remains on duty.
Spokeswoman Lt. Myra Taylor said Wednesday she can't "get into the particulars" of what prompted the state police to rescind Joyner's permission to work for Roethlisberger, except to say it was the result of state police brass "reviewing the initial report provided to us by Georgia authorities."
"If what you initially read is troubling, you have an obligation to take action even after that preliminary review," Taylor said.
Roethlisberger, a two-time Super Bowl winner and the highest-paid player in franchise history with a $102 million contract, also is being sued in a civil case by a Nevada hotel employee who has accused him of sexually assaulting her in 2008.
Roethlisberger practiced Monday and Tuesday with the Steelers, but he did not talk with reporters.
Chris Mortensen is ESPN's senior NFL analyst. Adam Schefter is ESPN's NFL Insider. Information from The Associated Press was used in this report.