CDD beat Soul, WCRS and the Norfolk agency Fox Murphy to win the £1 million account in a pitch called at the beginning of the year.
The appointment is a key part of a strategy to reinforce the Adnams claim that it is the fastest-growing beer brand in the UK and to secure a position as one of the top ten ales over the next three years.
Simon Loftus, the chairman of Adnams, said: "All the shortlisted candidates had exciting achievements to their credit and it was a tough choice, but the unanimous decision of the team was to appoint CDD."
He added: "The agency's presentation showed the best combination of strategic insight, brand vision and creative flair."
The strategy has yet to be finalised, but it is thought that a national press and poster campaign will break this summer. Dave Dye, the creative director at CDD, said: "Adnams is a genuinely premium brand with ambition and we are thrilled to be part of its plans."
Media strategy will also be handled by CDD and will aim to push the brand into the national market. The brewery is planning to bolster its national distribution as part of its growth plans.