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 on their website (www.thefa.com)