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.
Goodell said the league's conduct policy gave him the right to impose discipline regardless of whether he broke the law.
"I recognize that the allegations in Georgia were disputed and that they did not result in criminal charges being filed against you," he said in his letter to the two-time Super Bowl winner, a six-year veteran.
"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."
Roethlisberger must undergo a "comprehensive behavioral evaluation by medical professionals" and may not attend any team offseason activities until that evaluation is completed.
The suspension could be reduced to four games for good behavior. Sitting out all six games would cost him an estimated $2.8 million.
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 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.
"Your conduct raises sufficient concerns that I believe effective intervention now is the best step for your personal and professional welfare," Goodell said in the letter. "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."
Roethlisberger also is being sued by a woman who accused him of raping her at a Lake Tahoe hotel-casino in 2008. He denied the allegation and wasn't charged.
Goodell has aggressively dealt with players who violated the personal conduct policy throughout his 3½ years as commissioner. He banned Adam "Pacman" Jones for one year, and suspended Tank Johnson and Chris Henry eight games each in 2007.
Last year, Michael Vick was suspended for six games, later shortened to two games, for his role in a dogfighting ring. Vick also spent 18 months in jail.
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.
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 Art Rooney II said the team would have imposed sanctions on Roethlisberger before now, but doing so would have permitted the players union to appeal any penalties. Any suspension imposed by Goodell can be appealed only to him.
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.
Milledgeville is a central Georgia college town about 30 miles from where the quarterback owns a lake home.