Souk Communications held off competition from Kitcatt Kohr Alexander Shaw and Clark McKay & Walpole to win the business.
It will now create a fully integrated campaign with activity to include radio, press and online for Thorntons' direct sales division.
Lysanne McCallion, general manager of Thorntons Direct, said: "Souk has the integrated expertise, combined with strong creative, which we need to relaunch our brand in an ever-increasing competitive market."
The campaign, which is expected to launch in the autumn, will partially focus on developing Thorntons Direct's corporate and B2B marketing accounts.
Souk Communications' clients include Tesco, Emirates, Sony, Novell and Interflora.
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