Student Edward Sanoh must pay £290 after setting out to defend a friend from a late night attack.
He threw a punch at an unknown man when trouble broke out - but failed to connect, a court heard.
His friend had been struck in the face and Sanoh said that he only intervened to protect him.
After failing to strike the man, Sanoh was accused of running after him, then confronting him in the doorway of the Nationwide on Clumber Street.
But he told city magistrates: "I didn't chase him. He threw a punch towards me.
"At that point I punched back to try and defend myself. My back was to the wall," said Sanoh, a quantity surveying student in his last year at Nottingham Trent University.
He told the court that he had been to a bar and then Pryzm night club with the friend, who also studies at the university. When asked about effect of the alcohol he had downed, he said: "It doesn't make me do anything crazy."
But Neill Fawcett, prosecuting, asked: "Is it simply retribution because you would look out for one another?"
Sanoh, 22, replied: "It was in defence of a friend."
Ben Brown, defending, told the court: "There is no suggestion he was involved in whatever sparked this off.
"We don't even have a victim to this offence. You would expect him to go to the police and say 'this gentleman has thrown two punches at me. I want something done.'"
People have a legal right to defend themselves and other people, added Mr Brown, pointing out that Sanoh had never been in trouble before.
Sanoh of Hound's Gate pleaded not guilty to assaulting an unknown man by beating at around 4am on May 1, a students' night in the city. He was convicted and fined £60 with £230 costs.
Police sergeant Jodi Leonardi said that he spotted a disturbance involving up to 20 people near McDonald's.
He parked his van and ran down Clumber Street, telling the three JPs: "I saw Mr Sanoh swing his arm and hit someone in the head.
"I was two or three metres away. I got in the middle and held him back away from the situation."
