Shop's first task is expected to be a Christmas campaign to boost sales in the run-up to the festive season.
The account was held by Miles Calcraft Briginshaw Duffy, which parted ways with the chocolate-maker when it declined to repitch for the business.
The advertising review is part of a strategic rethink at Thorntons, which is aimed at revitalising the company's performance. It was instigated by Mike Davies, who joined as the chief executive in October 2006 from Mars.
Last month, Peter Wright was appointed as Thorntons' marketing director from Tesco Ireland, where he held a similar position.