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.

The figures come ahead of Wednesday's AGM where investors will quiz management over their plans for the Hinkley Point nuclear plant in the UK.
EDF, 85% controlled by the French state, has struggled to find the cash for its 66.5% stake in the project.
In April it pushed a final decision on the £18bn plant back to September.
Chief financial officer Thomas Piquemal resigned in April following an internal disagreement over whether to press on with the controversial project.
However, EDF has outlined plans to raise €4bn, with up to €3bn provided by the French government.
Credit rating agencies are due to assess the group in the coming days and their verdict on its finances will determine how easy it will be for the group to raise cash.
Meanwhile, tough market conditions mean EDF is cutting costs and planning to sell €10bn in assets by 2020, including a stake in French power-grid operator RTE.
On Tuesday the new chief financial office Xavier Girre, said the management was not considering spinning off any part of the group to raise money.
Its latest earnings report shows it has reduced its target for 2016 nuclear output to 408-412 terawatt-hour from 410-415.
The scaling down of its output reflects continuing problems at its Paluel nuclear facility in France where a steam generator collapsed during maintenance.
Sales fell 7% to €21.4bn in the first three months of the year, with sales in the UK down 9.8%.
The company said its earnings forecast for 2016 remained on track.

Services from Douglas to the UK were disrupted after the Ben-my-Chree, which sailed from Heysham, Lancashire, struck the pier on Sunday.
The Isle of Man Steam-Packet Company confirmed no passengers or crew were injured.
Four crossings between Douglas and Heysham were been cancelled following the the collision.
Chief executive Mark Woodward apologised to passengers and said it was an "unfortunate incident", adding the company would "do all we can to minimise the disruption".
A company spokesman said that on arrival in Douglas Harbour, the Ben-my-Chree "encountered significantly stronger than forecast easterly winds" and the vessel made contact with the King Edward Pier.
He said engineers worked through the night to assess any damage and a diving inspection had been carried out.
He added the Ben-my-Chree would return to service later, though its sailing at 19:45 GMT would be subject to "prevailing weather conditions".
Former Burnley midfielder Joey Barton's suspension from football for breaching betting rules has been reduced by almost five months after an appeal.
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French energy giant EDF says sales fell 7% in the first three months of the year in the face of stiff competition, a mild winter and lower energy prices.
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A ferry crashed into a pier on the Isle of Man as the captain tried to dock in strong winds.