Moments after his slam dunk in Sunday's game, World Peace was headed back upcourt when he ran into Harden. After pounding his chest in celebration, World Peace raised his left elbow and hit Harden flush, just behind his left ear toward the back of his skull.
Harden fell to the court and stayed down for about a minute and missed the rest of the game and World Peace was ejected.
"The concussion suffered by James Harden demonstrates the danger posed by violent acts of this kind, particularly when they are directed at the head area," NBA Commissioner David Stern said.
"We remain committed to taking necessary measures to protect the safety of NBA players, including the imposition of appropriate penalties for players with a history of on-court altercations."
World Peace, who changed his name from Ron Artest last September, has been suspended 13 times in his career for total of 111 games.
He will miss the Lakers' regular-season finale on Thursday and serve out the rest of his ban over the next six games he is eligible and physically able to play, including this season's playoffs.
The Lakers, who are 41-24 this season, said World Peace has for the most part been a model citizen both on and off the court since joining the Lakers in 2009 but that his behaviour on Sunday was not acceptable.
"His actions could have seriously injured another player, and his absence during this suspension will hurt our team as well," Lakers General Manager Mitch Kupchak said.
"While we accept the league's decision, we will be supportive of Metta and try to help him be more professional on the court."