Gaymers Original Cider was launched last year by the Gaymer Cider Company, the second-largest cider producer in the UK.
The agency will create a fully integrated campaign for the brand - including above-the-line advertising - which is part of a wider marketing investment of £26 million in all Gaymer Cider Company products.
As well as Gaymers Original Cider, brands owned by the Gaymer Cider Company include Blackthorn, Olde English, Gaymer's Orchard Reserve and Addlestones.
Paul Shearer, the global creative director at Nitro, said: "This is brilliant news for us, and comes off the back of a good run of new business. It proves that our integrated approach to communications is working. Gaymers is a brand that's ambitious, and wants to drive sales through great work, both of which we are excited about working closely to deliver."
John Mills, the managing director of the Gaymer Cider Company, said: "Nitro London stood out for its creativity and clear genuine passion and enthusiasm for Gaymers Original."