PriceRunner decided to appoint Greenlight to focus initially on its English-language UK and US sites, with a planned roll-out of the campaign to other European markets.
Greenlight will assist PriceRunner with both search engine optimisation and link building.
Matthias Berg, UK manager of PriceRunner, said: "As an international online business with many keywords and products listed on our site, we recognise the importance of an effective SEO campaign."
Warren Cowan, chief executive of Greenlight, added: "A strong SEO campaign is needed to reach the top of popular search engines in order to make PriceRunner easier for consumers to locate."