Goodell determined that Roethlisberger had violated the NFL's personal-conduct policy and ordered the two-time Super Bowl-winning quarterback to undergo a "comprehensive behavioral evaluation by professionals."
Goodell handed down the punishment one week after prosecutors decided not to charge Roethlisberger in a case involving a 20-year-old female college student who accused him of sexually assaulting her in a Milledgeville nightclub last month.
"I recognize that the allegations in Georgia were disputed and that they did not result in criminal charges being filed against you," Goodell said Wednesday in a letter to Roethlisberger. "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."
Goodell said Roethlisberger cannot participate in the Steelers' offseason activities until he completes the evaluation and is cleared by the league.
"Your conduct raises sufficient concerns that I believe effective intervention now is the best step for your personal and professional welfare," Goodell wrote.
Sitting out all six games would cost Roethlisberger an estimated $2.8 million, though the penalty could be shortened to four games for good behavior. Goodell said he will review Roethlisberger's progress before the start of the regular season and decide whether or not to reduce the suspension.
"I believe it is essential that you take full advantage of the resources available to you," Goodell wrote. "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."
Roethlisberger will not be eligible to play in games against the Atlanta Falcons (home), Tennessee Titans (road), Tampa Bay Buccaneers (road) and Baltimore Ravens (home) to open the season. The Steelers have a bye in Week 5. If the suspension lasts the full six games, Roethlisberger also would miss a Week 6 home game against the Cleveland Browns and a Week 7 road contest against the Miami Dolphins.
Roethlisberger is the first player who hasn't been arrested or charged with a crime yet received a suspension from Goodell. The commissioner said the league's conduct policy gave him the right to punish Roethlisberger, regardless of whether or not the quarterback broke the law.
"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 wrote. "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."
In a statement to police, the 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 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 the woman's 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 the woman's friends said they saw a bodyguard lead her into the hallway, then saw Roethlisberger follow. They said they couldn't see their friend but knew she was drunk and were worried about her.
The statements were among hundreds of pages of the case file made public last week by the Georgia Bureau of Investigation.
Roethlisberger also is being sued by a woman who accused him of raping her at a Lake Tahoe hotel-casino in 2008. He has denied the allegation and wasn't charged.
"In your six years in the NFL, you have first thrilled and now disappointed a great many people," Goodell wrote. "I urge you to take full advantage of this opportunity to get your life and career back on track."
Before acting, Goodell said 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 privately talked with Georgia district attorney Fred Bright. In addition, Goodell said he listened to recommendations from the quarterback's representatives and took into account information that the NFL office learned regarding the alleged assault.
The Steelers said Roethlisberger wasn't available for comment. His agent, Ryan Tollner, said he planned to respond later.
Steelers president Art Rooney II told reporters in a conference call Wednesday that he supported the punishment handed down by Goodell. Rooney said the team conducted its own investigation and also had access to all the information that the league gathered.
"When the commissioner acts under the league conduct policy, it is the commissioner's decision," Rooney said. "Clearly, in this case, we had an opportunity to have input in a number of conversations about what was going to happen. We certainly were able to coordinate on what was the final outcome."
Since Roethlisberger's suspension is the Steelers' second under the league's personal-conduct and drug policies, the team must pay 25 percent of the suspended player's forfeited salary to a maximum of $200,000. Wide receiver Santonio Holmes received a four-game suspension for violating the substance-abuse program before the Steelers traded him to the New York Jets last week.
The Steelersprepared for Roethlisberger's suspension Tuesday by acquiring quarterback Byron Leftwich from the Tampa Bay Buccaneers in exchange for a seventh-round draft pick (225th overall). Leftwich was Roethlisberger's backup during the Steelers' 2008 Super Bowl championship season and could be a fill-in starter.
"At this point, we felt like we would have Ben out for at least the first few games of the season," Rooney said. "We really didn't know for sure how many games until this morning. But we felt like we needed to add a vet to our quarterback position because of that. Byron has been here two years ago and had some success. He was a pretty obvious choice to bring back in."
Goodell has aggressively dealt with players who violated the personal-conduct policy throughout his 3½ years as commissioner. He banned then-Tennessee Titans cornerback Adam "Pacman" Jones for one year and suspended then-Chicago Bears defensive tackle Tank Johnson and Cincinnati Bengals wide receiver Chris Henry eight games each in 2007. Henry died last year of a head injury after falling off the back of a truck.
In all, 16 players have been suspended under the conduct policy by Goodell.
Rooney said he doesn't expect the NFL Players Association to appeal Roethlisberger's suspension.
"My understanding is that the commissioner has discussed this with (NFLPA President DeMaurice) Smith," Rooney said. "I really would hesitate to speak for either one of them, but it is my understanding that the players association does not plan to appeal it. But obviously they need to review, especially with Ben."
The Associated Press contributed to this report.