Manhattan Judge Denise Cote said the iPad maker "conspired to restrain trade". But the firm's spokesman, Tom Neumayr, said Apple would appeal against the ruling and fight "false allegations".
Five publishers that were originally named as defendants alongside Apple have already reached settlements, including Penguin. The judge ordered a new hearing to determine damages to be imposed on Apple.
The US Department of Justice said the conspiracy was designed to challenge online retailer Amazon's dominance of the fast-growing e-books market.
Penguin settled its case for $75m (£49m). Hachette, HarperCollins and Simon & Schuster created a $69m fund for refunds to consumers, while Macmillan settled for $26m.
Judge Cote said: "The plaintiffs have shown that the publisher defendants conspired with each other to eliminate retail price competition in order to raise e-book prices, and that Apple played a central role in facilitating and executing that conspiracy. Without Apple's orchestration of this conspiracy, it would not have succeeded as it did in the spring of 2010," she said.
US Assistant Attorney General Bill Baer called the ruling "a victory for millions of consumers who choose to read books electronically". He said the judge agreed with the Justice Department and 33 state attorneys general that executives at the highest levels of Apple orchestrated a conspiracy with five major publishers to raise prices.
Source: BBC News
