Inferno won the business after a pitch and will work on a campaign spanning TV, outdoor, print, trade, point-of-sale, digital and social media.
It will develop a strategic repositioning of Merlin’s pan-European Dungeons portfolio of attractions, where local history is brought to life with actors and special effects.
The appointment follows the announcement that The London Dungeon is to move to a new site at County Hall.
Rebecca Hamilton, the global head of marketing at Merlin, said: "Inferno gave us an impressive pitch presentation and, as such, we’re delighted to be working with them. 2013 will be an exciting year for The Dungeons, with a bigger, better, bolder new London Dungeon opening next to the London Eye in March and a Dungeon opening in Berlin.
"The Dungeons offer such a fantastic experience that leaves you laughing – we needed to work with an agency that could communicate the experience as well as ‘get’ our sense of humour."
Tim Doust, the Inferno vice-chairman and founder, said: "We are thrilled to be working with such a well-loved brand as The Dungeons."
Separately, Merlin kicked off a pitch earlier this month to find an agency to create a 2013 campaign for the EDF Energy London Eye.