Extortionists have demanded a ransom from game developer CD Projekt Red for the safe return of files for the studio's next game.
Unless the Polish company pays up, it risks files being made public. The size of the ransom demand is not known.
The game-maker said it would not be "giving in to the demands" of the gang.
It added that the documents that had been taken were old and "largely unrepresentative" of the current state of the game.
It is not clear how the gang demanding the ransom got hold of the documents.
CD Projekt has not given any more details about the theft beyond the statement it put on its official Twitter account.
Best known for the Witcher series of video games, CD Project said it had told the police about the ransom demand.
The studio first announced that it was working on a cyberpunk-themed game in 2012.
No definite release date for of Cyberpunk 2077 has yet been given, but the game is expected to be launched before 2021.
Computer security expert Graham Cluley said CD Projekt should be "applauded" for its refusal to pay the ransom.
"Paying extortionists always runs the risk of encouraging blackmailers to strike again, putting not just your own company but others at further risk," he said.
Mr Cluley said the attack was just the latest in a series that had held intellectual property to ransom.
Last month, a group called the Dark Overlord sought to blackmail Netflix over unaired episodes of a popular TV series.