The move, which comes as Yahoo loses ground to rivals in online advertising, will use comScore's verification platform to provide better data to marketers.
"Brands are reaching their audiences through multiple channels and need a consistent way to measure results across their campaigns," said Eric Lange of Yahoo.
Lange added that the new service "provides advertisers with one place to buy, measure and analyze campaigns against all available data. That's something advertisers can't get anywhere else today."
While online advertising is surging, Yahoo has failed to keep pace with juggernauts such as Google and Facebook, according to analysts.
The research firm eMarketer said last month that Yahoo ad revenues fell 2.1 percent in 2013, and will grow a modest 2.7 percent this year.
But the report said Yahoo will be overtaken by Microsoft for third place in online ad revenues, behind market leader Google and second-place Facebook.
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