The 19-year-old was issued with the fixed penalty notice after ignoring warnings to shut down the party at his Nottingham home on Friday night.
In a letter sent to his neighbours, he said the “foolish gathering” was a “major lapse of judgement”.
Nottinghamshire Police said the group was breaking social distancing rules and the fine should serve as a warning.
It said about 50 people found at the address on Harlaxton Drive, in Lenton, refused to leave and the host was issued with the fine.
he party host wrote in the letter that friends were hoping to have a party of 25 people – adhering to Covid-19 restrictions – to celebrate two housemates recently turning 21.
“However, it quickly became out of hand,” he said.
“It was never our intention to disrupt your evening. It was a major lapse in judgement on our end.
“We are eager to make amends.”
The force said it used its full powers to deal with the “reckless” organiser who “deliberately flouted” the rules.
But one neighbour told the BBC the £10,000 fine was “absolutely ridiculous… fine the guys, but we were all students once”.
Another resident said there had been large student parties in the Lenton area during the coronavirus pandemic and hoped lessons had been learned.
Chief Constable Craig Guildford said: “The individual had their chance and that chance wasn’t heeded.
“The officers acted correctly and proportionately.
“In extremes, if people aren’t following the rules, after we’ve tried our best… we will enforce and issue fines as a last resort.”
People were warned not to treat this weekend as a “party” after one scientist warned the UK was “on the edge of losing control” of the virus.
The new “rule of six” coronavirus restrictions, limiting gatherings to six people indoors and outdoors in England, come into force on Monday.
Read from source: https://www.bbc.com/news/uk-england-nottinghamshire-54140150