Yahoo Inc. said in a court filing Monday that Facebook has infringed 10 of its patents covering advertising, privacy controls and social networking.
Yahoo threatened to sue Facebook last month, insisting that the social network license its patents.
Facebook pledged to defend itself vigorously against what it called "puzzling actions" by Yahoo.
"We're disappointed that Yahoo, a longtime business partner of Facebook and a company that has substantially benefited from its association with Facebook, has decided to resort to litigation," Facebook said in a statement.
The patent claims could cast a spotlight on Facebook's vulnerabilities as the company tries to complete an initial public offering of stock this spring. At the end of 2011, only 56 U.S. patents had been issued to Facebook. That's a relatively small number compared with other big tech companies. Yahoo, on the other hand, owns more than 1,000 patents.
Yahoo defended its lawsuit, saying it has invested "substantial resources in research and development" over the years that have led to patents of technology that other companies have licensed.
"Unfortunately, the matter with Facebook remains unresolved," the company said in a statement.
Yahoo, which has seen its revenue fall steadily over the past three years, made hundreds of millions of dollars from a patent settlement that it reached with Google Inc. just before that Internet search leader went public in 2004.