Dappy, whose real name is Dino Costas Contostavlos, denied attacking George Chittock in the early hours of 27th February at Chicago's nightclub in Chelmsford.
Dappy had initially insisted that he lashed out in self-defence after being told to go and "suck your dead dad" by the man.
The N-Dubz singer claimed George Chittock had abused him three times over the course of the night, including insulting his dead father.
"He looked like he was going to do something," he said, adding, "It was self-defence, I did not want to get hit."
However, when summing up the case, the lawyer for the prosecution told the court: "It is no defence for anybody, celebrity or otherwise, to say 'I lashed out because he was rude to me'."
The chair of the bench revealed that magistrates at Chelmsford Magistrates' Court had decided Dappy Contostavlos was not under any physical threat at the time of the incident, finding him guilty of assault.
Holding his head in his hands, Dappy listened as he was told that he would have to pay a fine of £800.
As well as the fine, he was ordered to pay a victim surcharge of £80 and costs of £930.