The 34-year-old's ban now expires on 1 June 2018, instead of 25 October 2018.
He was banned in April after being charged with breaking FA rules for placing 1,260 bets on matches between 26 March 2006 and 13 May 2016.
Barton, who says he is addicted to gambling, appealed against the length of the ban which he called "excessive".
The FA's Appeal Board said the initial ban was "excessive in the circumstances" as evidence from Barton's consultant psychiatrist about his addiction should not have been rejected.
The report says the new ban "reflects the overall seriousness of the breaches and also the mitigation including the full extent of Mr Barton's addiction".
Barton, whose fine of £30,000 remained unchanged, was released from Burnley in May.
Players in England's top eight tiers are banned from betting on football.
Former Manchester City and Newcastle midfielder Barton rejoined Burnley in January, having left Scottish Premiership side Rangers in November.
In November 2016, he was given a one-match ban for breaking Scottish Football Association rules on gambling.
Barton admitted the Scottish FA charge of placing 44 bets between 1 July and 15 September 2016, while he was a player at Ibrox.
Former Burnley midfielder Joey Barton's suspension from football for breaching betting rules has been reduced by almost five months after an appeal.