Tin Man has won a place on the roster after initially working for the food company on a number of individual brand campaigns.
The agency will now work on multiple UK campaigns, including brand equity, education, new product launches and seasonal retail support.
Tin Man founder Mandy Sharp said: "This means we can single-mindedly focus on building deeper emotional engagement and understanding with consumers, particularly the core parent target audience."
Ferrero UK’s brands include Kinder Surprise, Ferrero Rocher, Nutella, Tic Tac and Thorntons.
Charlie Cayton, director of communications at Ferrero UK, said: "We were looking to add to our roster of agencies with a new agency that is able to deliver strong strategic consumer comms campaigns at a pace that matches our ambitions."