The St James' Park club confirmed on Monday afternoon that the 28-year-old, who still has a year of his contract left to run, can leave with immediate effect.
The move comes after another prolonged Twitter outburst from Barton, who has become increasingly disgruntled with off-the-field matters in the north-east.
He took to the social networking site yesterday afternoon to hint at behind-the-scenes unrest, and has seemingly tipped owner Mike Ashley over the edge in the process.
Barton cryptically said earlier today that he would make an announcement about his future at 4pm, but the club pre-empted that by an hour and revealed he was able to leave, with a short statement saying: "Newcastle United can confirm that Joey Barton has been placed on the transfer list today.
"The player has been advised that he can leave the club on a free transfer."
In response, Barton tweeted 55 minutes later than expected, getting in another dig at the board.
He wrote: "Somewhere in those high echelons of NUFC, they have decided, I am persona non grata.
"I am on a free but the honour of wearing those B+W stripes, surpasses that.
"One day the board might realise, what the shirt signifies. HONOUR and PRIDE. Thanks for your continued support. £toonarmy."
The root of Barton's unrest can be traced back to the sale of Andy Carroll to Liverpool in January.
It was thought that the one-cap England midfielder was considering signing a new deal at the time, but decided to put his future on hold until the summer after that.
Then, in May, his agent Willie McKay said Barton would not be offered a new deal, with the player later tweeting that Newcastle wanted a "younger, better, cheaper player".
His discontent was not aided by the sale of close friend and club captain Kevin Nolan to West Ham, and following yesterday's 3-2 friendly defeat at Leeds he took another swipe at club officials.
In the aftermath of full-back Jose Enrique being fined for questioning the club's ambition, he said: "If only we as players could tell the fans exactly how it is, without them above fining us lots of money. There will be a time and a place.
"If it wouldn't effect team morale and cause unrest within the dressing room, am certain Jose's comments would be the tip of the iceberg.
"And again it would be left to those magnificent fans to pick up the remnants of their once great football club. £hadenoughofcertainpeople.
"If I wanted to leave, I'd just come out and say I want to leave. Things need addressing as am not prepared to go through a relegation again."
An acrimonious departure now seems inevitable, but Barton is likely to leave as a hero in the eyes of the fans having previously been a villain.
After joining for £5.8million from Manchester City in June 2007 he served a 77-day prison sentence after being found guilty of assault following an incident in Liverpool, while he was also given a suspended four-month sentence for a training ground assault on then City team-mate Ousmane Dabo.
The Dabo incident earned him a six-match ban upon his release from jail, before injury then curtailed his first-team comeback, and his days at the club looked to be numbered as he was sent off in a late-season clash at Liverpool - a game Newcastle lost 3-0 on their way to relegation from the Premier League.
He clashed with manager Alan Shearer in the aftermath and was suspended by the club, but resolved his differences sufficiently enough to remain with them in the Championship under new boss Chris Hughton.
Although injury ruled him out for the middle part of the season he played key roles at the start and end of what was a title-winning campaign.
He went on to become a key figure in the first team last season, scoring five goals in 35 appearances but, having been so outraged at the sales of Carroll and Nolan, he now looks set to join them in playing his football elsewhere.