The QPR captain opted not to press charges after being involved in an incident in Liverpool city centre on Monday, but has since been held on suspicion of affray
Two men were also arrested at the scene for a public order offence after they allegedly hit the 29-year-old in the head.
Barton was with his girlfriend in Liverpool city centre when the incident occurred outside Garlands nightclub in the early hours of Monday morning.
The former Newcastle player was held and questioned before being arrested, along with a 20-year-old man, on Monday afternoon.
A spokesman for Merseyside Police told reporters: "Following an altercation in Eberle Street, Liverpool city centre, in the early hours of this morning, four men have been arrested on suspicion of affray.
"Two 21-year-old men were arrested at the scene in the early hours of this morning and another two men, a 20-year-old from the Liverpool area and 29-year-old from the Widnes area, were arrested this afternoon.
"The two 21-year-old men and the 20-year-old man have been questioned by officers and have been released on bail pending further inquiries."
QPR spokesman Ian Taylor said in a statement: "Barton was on his way home from a night out with his girlfriend when two men sang offensive chants and then proceeded to hit the QPR midfielder."
Barton also described what happened via Twitter, suggesting that that the incident was not serious and revealing he had no plans to press charges.
He wrote: "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 it's 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."
A police statement released prior to Barton being released on bail read: "The 29-year-old man remains in police custody where he will be questioned by detectives."
Barton is already under investigation by his club after the FA handed him a 12-match ban for his actions during his side's Premier League defeat to Manchester City on the final day of the season.
He was sent off for elbowing Carlos Tevez before kicking out at Sergio Aguero and trying to headbutt Vincent Kompany.