The Social Network, which has been directed by David Fincher, tells the story of the founding of Facebook.
Reznor said he agreed to score the film after Fincher approached him with the idea last year.
“I’ve always loved David’s work but quite honestly I wondered what would draw him to tell that story,” he said.
“When I actually read the script and realized what he was up to, I said goodbye to that free time I had planned.”
Reznor has been writing the soundtrack alongside Atticus Ross, and said the pair were near the end of the process.
“We’re looking forward to the next phase – distilling the large amount of music we’ve written for this down to a satisfying record (or two),” he said.
The Social Network is due to be released in cinemas in the US on October 1.