Thorntons has hired Nexus Communications after a five-way pitch that included incumbent Salt PR.
The brief is thought to be worth between £3,000 and £7,000 a month.
Nexus will cover all aspects of consumer and trade PR, with Nudge PR continuing to serve as Thornton's chocolate specialist agency dealing with the high-end food media and supporting the retailer during Chocolate Week (8-14 October).
Thorntons PR manager Emma Tagg said: ‘Coming out of our centenary year we have a full calendar of chocolate events and stories to tell that will aim to put Thorntons front of mind for many occasions.
‘We want Nexus to get creative and build fresh affinity with both current and future fans, and help to "make everyday special".'
Nexus is also charged with the running of the Thorntons press office, but not its social media activities, which will remain in-house.
The 100-year-old chocolatier issued three profit warnings last year and plans to close up to 180 stores during the next three years.
The retailer saw like-for-like sales fall by 1.6 per cent in the 16 weeks up to 28 April. The retailer is also forecast to make a full-year loss of £2.4m.
It has faced increasing competition from supermarkets and premium brands like Hotel Chocolat on the high street.