An injunction issued by the US district court in New York has effectively shut down LimeWire, one of the internet's biggest file-sharing sites.
It ends four years of wrangling between the privately-owned Lime Group and the Recording Industry Association of America (RIAA).
The injunction compels Lime Group to disable its searching, downloading, uploading and file trading features.
The firm plans to launch new services that adhere to copyright laws soon.
Visitors to the LimeWire website are confronted with a legal notice that reads: "This is an offical notice that LimeWire is under a court ordered injunction to stop distributing and supporting its file-sharing software."
It adds that "downloading or sharing copyrighted content without authorisation is illegal".
The RIAA told the AP news agency that it was pleased by the judge's decision.
"It will start to unwind the massive piracy machine that LimeWire... used to enrich themselves immensely," said RIAA spokesman Jonathan Lamy.
LimeGroup appeared to acknowledge defeat.
"We are out of the file-sharing business, but you can make it known that other aspects of our business remain ongoing," Lime Group spokeswoman Tiffany Guarnaccia told AP.
The firm is working on developing new software that will adhere to copyright laws.