The Football Association confirmed an independent regulatory commission found Barton guilty of two counts of violent conduct on Wednesday and say he has "tarnished the image of football".
The FA announced Barton received an automatic four-game suspension, with the additional eight matches a consequence of the manner of his dismissal. He was also fined £75,000.
Barton was shown a red card at the Etihad Stadium for elbowing Carlos Tevez, and he then kicked Sergio Aguero before attempting to headbutt Vincent Kompany.
FA: "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.
"An independent regulatory commission has today dealt with two charges of violent conduct against Joey Barton, arising from the game against Manchester City, resulting in an eight-match suspension and £75,000 fine for the Queens Park Rangers midfielder.
"This eight-match suspension is to run consecutively to the four-game suspension Barton was already given for his dismissal in the fixture, making a total of 12 matches.
"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.
"Barton is already serving a four-match suspension for his dismissal in the match, which consists of three matches for violent conduct plus one match as this was the player's second dismissal of the season.
"The two further breaches for violent conduct ordered he be suspended for eight matches to run consecutively to the four matches, taking the total suspension to 12 matches."