The controversial Queens Park Rangers midfielder was involved in a fight with two men outside Garlands in Liverpool city centre.
A picture of the incident, apparently showing a shirtless and bloodied Barton being held back by police and a woman, appeared on Twitter.
Writing on the social networking site today Barton revealed: "I got sucker punched in the back of the head.
"Not injured as reported just a small bruise on rear of head/ear.

"I've had worse. Life goes on! Thanks for the concern. Have a good day people.
"Not planning on pressing charges as its not that serious. The police have also got better things to do with their time.
"Can I just say a public thanks to the police last night as they were and have been today brilliant."
Merseyside Police were called to a disturbance outside the popular nightclub at 5.30am.
They confirmed a 29-year-old man had been injured. Two 21-year-old men were arrested for a public order offence and are currently in police custody.

QPR midfielder Barton, 29, was banned for 12 matches after being sent off during his side's defeat at Manchester City on the final day of the season. He elbowed one Man City player, kicked out at a second and tried to headbutt a third.
The Football Association found him guilty of two counts of violent conduct.
In 2008, Barton was given a six-month jail term after admitting assault and affray during an incident in Liverpool. He joined QPR in August last year on a free transfer from Newcastle United.
Club spokesman Ian Taylor said Barton was on his way home from a night out with his girlfriend when two men began singing offensive chants before a fight started.
He added: “Police were immediately on the scene. Barton decided not to press charges and left the scene with his girlfriend