Saltmarsh will start work for the Channel Island and its sister islands Alderney, Herm and Sark, on 1 January.
Visit Guernsey joins Saltmarsh's existing portfolio of eight tourism authorities, as well as tour operators, hotels, airlines, and other business and leisure travel clients.
The six-strong account team is led by managing director Geoff Saltmarsh, with day to day offline activity headed by director David Ezra. A separate dedicated online campaign will be run by digital director Rebecca Genin.
The Saltmarsh Partnership's winning pitch was based on the growing consumer trend of a return to traditional values of seeking goods and services reminiscent of childhood, which is now increasingly being reflected in holiday choices.
The brief includes increasing first-time visits to Guernsey, as well as continuing to attract its core markets of couples and families with young children.
Guernsey's director of marketing and tourism Chris Elliott said: ‘We have wonderful scenery, outstanding restaurants, fascinating heritage attractions and a range of activities. This means that our visitors can "potter" or be active and we are confident that the new PR campaign will ensure we appeal to holidaymakers who want traditional breaks of great quality and excellent value.'