Uber has been told to return thousands of "pilfered" files to Alphabet's Waymo self-driving car division.
The San Francisco judge also banned a top Uber engineer from involvement in certain work for the ride-hailing firm's autonomous driving project.
Waymo has sued Uber over claims that a former employee stole trade secrets that were later used by its rival.
Although a blow to Uber, the ruling stopped short of ordering a shutdown of the firm's self-driving unit.
District Judge William Alsup's ruling, which was unsealed on Monday, said Waymo "has shown compelling evidence that its former star engineer" Anthony Levandowski "pilfered" thousands of documents from the company.
"The bottom line is the evidence indicates that Uber hired Levandowski even though it knew or should have known that he possessed over 14,000 confidential Waymo files likely containing Waymo's intellectual property," Alsup wrote.
Uber must return the documents either to Waymo or the court by the end of this month.
The judge also ordered Uber to keep Mr Levandowski away from work involving Lidar, a key sensor technology in self-driving cars that is the crux of the current litigation.
Waymo spokesman Johnny Luu welcomed the ruling, saying: "Competition should be fuelled by innovation in the labs and on the roads, not through unlawful actions."
However, the judge added that "few" of Waymo's alleged trade secrets have been traced to Uber's self-driving car technology. Not all of Waymo's 121 asserted trade secrets qualify as trade secrets, he added.
Crucially for Uber, the ruling did not shut down its self-driving car project. Uber is betting that its ride services network will eventually rely on self-driving cars.
"We are pleased with the court's ruling that Uber can continue building and utilizing all of its self-driving technology, including our innovation around Lidar," Uber spokeswoman Chelsea Kohler said.
On Monday Waymo signed a deal with Lyft, Uber's main US competitor, to test its self-driving technology.