James Bimson, 29, hit a member of door staff who had escorted him out of The Embassy in Broadway.
City magistrates were told on Monday that Covid rules were in place at the time, including guests having to remain seated at all times.
Prosecutor Pretty Barber said Bimson had become angry with staff trying to enforce the rules and threw “a couple of weak punches” at Ryan Sherlock.
Bimson was taken outside and initially walked away before heading back towards Mr Sherlock and hitting him in the face with an “overhand punch”, causing bruising.
The court was told Bimson, who pleaded guilty to the June 30 assault, had previous convictions in the last ten years for affray, assault and battery.
Julie Murtagh, mitigating, described the most recent offence as “a little bit of a perfect storm” for her client.
She said he had finished his second year at university and was back home in the city watching the football at the bar on one of the first occasions it had been open again.
His group of friends numbered 15, which was split over two tables, and trouble began because they were not keeping to their dedicated positions.
“The issue was that they felt the bouncers were not making other people comply in the same way,” she added. “The upshot was Mr Bimson was literally picked up and carried out and, in the process, a ring fell off.
“Outside there was a request to go back and find it which was rejected. It was then he got angry and threw the single punch.”
Bimson, of Brewer Street, Brighton, was fined £100 and ordered to pay £150 compensation to Mr Sherlock.