Commissioner David Stern said the substantial penalty was levied on World Peace partly because Harden sustained a concussion from the blow, which was delivered during the Lakers' double-overtime win Sunday.
"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," Stern said in a statement. "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."
Harden fell to the floor after being struck and World Peace was assessed a flagrant foul-2, earning an ejection from the contest.
The Lakers have one regular season game remaining, so the suspension will carry into the playoffs.
"Metta has for the most part been a model citizen both on and off the court since joining the Lakers," Lakers General Manager Mitch Kupchak said in a statement. "Still, his most recent lapse in judgment is not to be condoned or accepted. His actions could have seriously injured another player, and his absence during this suspension will hurt our team as well. While we accept the league's decision, we will be supportive of Metta and try to help him be more professional on the court."