The provides high school and college-level results and statistics on a host of US sports, including basketball, American football and baseball, and will be used by Yahoo! to build stronger communities among its users, increase advertising revenue and subscription business.
Yahoo's acquisition of Rivals.com follows Terry Semel stepping down as chief executive officer earlier this week. He will become non-executive chairman and has been replaced by Yahoo! co-founder Jerry Yang.
Rivals.com, which was founded in 2001, has a network of more than 150 websites and attracts around 2m sports fans every month.
Yahoo! said the acquisition was important to fulfill its remit of building online communities at a localised level, and will be used to complement the digital giant's existing Yahoo! Sports service.
Shannon Terry, chief executive officer of Rivals.com, said: "With the Yahoo! acquisition, Rivals.com is beginning the next stage in the evolution of our business. We will now have exposure to millions of additional sports fans, as well as marketing, sales and engineering support which were previously unavailable to our company."
Rivals.com, which is based in Tennessee, employs more than 300 sports writers, and markets itself as creating "the ultimate fan experience" for sports followers in the US, and has its stories syndicated to AOL Sports, MSNBC.com and USA Today.