According to the lawsuit, filed in Santa Clara County Superior Court in California, the unknown party posted digital images of Apple's dual-processor PowerMac and its new mouse to a publicly accessible site on the Internet, beginning "in or about" February.
In addition, Apple said the individual, alone or in concert with others, has posted trade secret information about other Apple products that the company has not announced.
Company executives did not elaborate on the suit. Apple is seeking an injunction against further disclosure of its trade secrets as well as monetary damages.
In the suit, Apple said it believes that the posting is "willful and malicious" and that it will continue causing the company to suffer "great and irreparable harm and damage."
Daniel Harris, a partner in the intellectual property group at Brobeck Phleger & Harrison, said the main purpose of these type of suits typically is to find out who is leaking the information.
By filing a lawsuit, Harris said the company can try to subpoena the Internet service provider or host of the web site where the information was posted.
"It gives you the power to gather information," Harris said.
Shortly before last month's Macworld Expo, several Macintosh rumor sites reported that they were told by Apple's legal department to take down alleged inside information about a cube-shaped computer, later revealed to be Apple's PowerMac G4 Cube.
Apple may be on the receiving end of a lawsuit related to the Cube, as server maker Cobalt Networks may be considering legal action. Cobalt makes a blue, cube-shaped server, dubbed the Qube.
The Cupertino, Calif.-based computer maker has also sued to protect its own designs, particularly the iMac. Several of those cases were settled earlier this year.
In the lawsuit, Apple claims that advanced knowledge of its future products hurts sales of current models and tips off competitors on Apple's strategy.
Harris said Apple would be able to show it was damaged if it is able to prove infringement.
"Is it hundreds of millions of dollars? Probably not," Harris said. "But would it be significant? Sure."
Whether Apple will pursue the lawsuit if it discovers the identity of the individual is unclear, Harris said. In some cases, companies file the suits to indentify the source of an internal leak and then choose to fire the worker but drop the suit.
Internet posting of company secrets has become a major issue for large corporations. Last year, Ford was embroiled in an action against the Blue Oval News Web site, which Ford alleged was posting inside information.