He was also fined £75,000.
Barton was sent off for elbowing Manchester City's Carlos Tevez and then kicked Sergio Aguero in QPR's final Premier League game of the season on May 13.
Barton accepted the charge of violent conduct against Aguero but denied the same charge against City defender Vincent Kompany, who he appeared to try and head-butt before being escorted off the pitch.
An Independent Regulatory Commission found Barton guilty of both charges and he will serve the eight-game ban consecutively with the four-match suspension he received for the sending-off, the FA said in a statement.
"Following Barton's dismissal in the 54th minute of the match, there followed two incidents involving Manchester City's Sergio Aguero and Vincent Kompany. As Barton had already been dismissed, both of these incidents fell outside the jurisdiction of the referee.
"Barton accepted the charge of violent conduct against Aguero but denied the second breach of violent conduct against Kompany. The independent regulatory commission, however, found this second charge proved."
The chairman commission Brian Jones slammed Barton for his actions.
"There are rules of conduct that should be adhered to, and such behaviour tarnishes the image of football in this country, particularly as this match was the pinnacle of the domestic season and watched by millions around the globe," he said.
Barton, 29, who has a long history of trouble on and off the field, had said on Twitter on the day of his sending-off that he had not lost his head, but that a team-mate "suggested I should try to take one of theirs with me."
He later expressed remorse but maintained he had been hard done by.
"Can do nothing but apologise to the players and the fans. Still don't think it’s a sending off."
QPR lost the game against City but remained in the Premier League by one point.
In May 2008 Barton was sentenced to six months' imprisonment for common assault and affray during an incident in Liverpool.
The troubled Englishman has a long history of violent incidents both on and off the pitch, with a training-ground fight with former team-mate Ousmane Dabo contributing to Barton departing Manchester City.