According to Comscore Media Metrix, Blue Lithium is the fifth largest ad network in the US and second largest in the UK, with 145m unique visitors each month.
Yahoo! said the acquisition would help it offer advertisers access to powerful data analytics, advanced targeting and innovative direct response buying strategies across a wide range of high-quality stock.
Five months ago Yahoo! bought another internet ad network, or exchange, Right Media, for $680m.
Susan Decker, president of Yahoo!, said: "Blue Lithium's expertise in network management will better enable Yahoo! to manage supply and demand across our network by balancing advertiser goals with publisher value."
The acquisition of the advertising networks is one of a number this year by big league players. DoubleClick went to Google in April for $3.1bn; 24/7 Real Media went to WPP Group for $649m and aQuantive went to Microsoft for $6bn.
Blue Lithium is based in San Jose, California, and has 100 employees.
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