Under the agreement, Reuters will sell all online advertising that targets the American audience of Guardian.co.uk. More than 5 million of Guardian.co.uk's 18 million monthly unique users are based in the US.
The Guardian will be sold alongside Reuters.com and the recently launched Reuters Affiliate Network, a growing collection of news sites and blogs with a targeted audience of business professionals.
"By adding the Guardian to Reuters' sales portfolio, we are continuing our efforts to provide a range of options for advertisers to access our growing audience of affluent business professionals," said Chris Ahearn, president of Reuters Media.
"Through Reuters' sales team, advertisers can more effectively reach our growing and discerning audience, and our U.S. users will be served with more relevant advertising," added Adam Freeman, commercial director of Guardian News and Media.