The deal, which replaces an existing nine-month partnership with Outrider, is part of an initiative by the travel company to streamline the number of agencies it uses and to create tighter links between its online and offline campaigns.
Since Opodo's launch earlier this year, Carat has been responsible for all offline advertising in Opodo's three live markets - the UK, Germany and France. Carat will now be responsible for all online media planning and buying as well as offline advertising for the firm.
"This move will deepen the relationship between Opodo and Carat, with a focus on integrated campaigns, and increase effectiveness of spend," said Jenny Taylor, UK country manager at Opodo. "Carat has proven pan-European experience, as well as a track record of integrated campaigns using all forms of media."
Opodo was launched with £78 million backing from nine European airlines to compete with travel aggregators such as Expedia and ebookers.