The NFL announced the suspension on Thursday. Lynch is expected to appeal the suspension.
Lynch will lose approximately $112,000 of his $635,000 base salary for 2009 during the suspension.
Lynch was arrested near Los Angeles on Feb. 11 and later pleaded guilty to a misdemeanor gun charge. In searching a parked car carrying Lynch, Culver City police found a 9 mm semiautomatic handgun inside a backpack in the trunk.
Prosecutors said the backpack contained other items belonging to Lynch. Police said they also found four marijuana cigarettes in the car, but no drug charges were filed.
After pleading guilty to having a concealed firearm in a vehicle, Lynch was sentenced last month to 80 hours of community service and three years' probation. The Bills' 2007 first-round draft pick also issued an apology.
"I am embarrassed by my recent arrest and conviction," Lynch said. "I deeply regret that I placed myself in this situation."
Lynch met with Goodell at his office in New York on March 17.
"We have been informed by the league as to Commissioner Goodell's decision regarding the suspension of Marshawn Lynch." the Bills said in a statement. "We fully support the NFL Personal Conduct Policy and believe that Marshawn is committed to conducting himself responsibly. When he spoke to the media recently, he talked about making changes and better decisions and we feel that he is moving forward in a positive direction."
It was Lynch's second brush with the law in less than a year. Last June, Lynch pleaded guilty to a traffic violation and admitted driving off after striking a female pedestrian with his car near Buffalo's downtown bar district May 31.
Goodell did not discipline Lynch for the traffic violation after meeting with him at training camp last summer.
Lynch had eight touchdowns and rushed for 1,036 yards last season. He went to his first Pro Bowl game last month, running for a game-high 48 yards during the AFC's 30-21 loss to the NFC.
Lynch already has become more active in the Buffalo community. Earlier this week, Lynch and Bills cornerback Terrence McGee provided financial assistance to help sponsor the 18th annual Gus Macker 3-on-3 basketball tournament, which will be held in Buffalo in late June.
"They are really doing something pretty historic," Buffalo Mayor Byron Brown said. "Not only are they devoting some of their time, but they are putting up their personal dollars to support the event."
Michael Smith covers the NFL for ESPN. Information from the Associated Press was used in this report.