The league said Thornton won't be eligible to return until the Bruins' road game against San Jose Jan. 11 and will forfeit $84,615 in salary for his actions in the Dec. 7 contest against the Penguins.
In the incident, Thornton pushed Orpik over backwards onto the ice while they were engaged in a scrum and punched him in the face while kneeling over him.
The Pens' defenseman was briefly unconscious before being stretchered off the ice and taken to a local hospital. He was diagnosed with a concussion this week and placed on injured reserve.
Thornton had already missed three games while awaiting the NHL's ruling.
Orpik earlier in the same game had delivered what was judged to a clean check against Boston's Loui Eriksson, which resulted in a concussion to the Bruins forward.