The appointment of McCann follows a review process that kicked off in August last year and involved a final three-way pitch.
Agency sources said McCann will handle the £3 million UK account plus the business in Australia and in selected European markets.
Work Club, the incumbent on the integrated UK business, did not pitch for the account but will continue to handle digital alongside the General Mills brands Häagen-Dazs and Old El Paso.
Nature Valley, which launched in the UK in 2006, was supported last year with distribution at London Underground stations to promote its status as the official supplier of cereal bars to London 2012.
Also in 2012, Nature Valley launched a TV ad by The Independence Orchestra showing city dwellers falling into a farmer’s field as they ate their cereal bars.
Rosie Doggett, who runs RAD Consulting, co-ordinated the pitch process.