BBH beat Wieden & Kennedy, the incumbent on GNM's ad business, and Rainey Kelly Campbell Roalfe/Y&R to win the brief.
Recent high-profile TV campaigns including the animated "gola mondo" work to promote The Guardian and The Observer’s coverage of the Fifa World Cup were created by Wieden & Kennedy.
The pitch follows a recent coverprice increase at The Guardian and the launch this week of its iPad edition.
Adam Freeman, the executive director, commercial, at GNM, said: "We're looking forward to working with BBH on an integrated brand and marketing strategy as we build on our growing international reputation as an open, independent and digital-first media organisation."
Jason Gonsalves, the managing partner at BBH, said: "At BBH we believe that creativity and a commitment to difference is what enables businesses to outperform competitors and redefine their categories. That's why we are so excited to be working with a pioneering organisation like the Guardian, to give voice to the brand at a time when it is evolving into a digitally powered, global business."