Karmarama won the account, which was previously with Faulds Advertising, after a six-way pitch.
Emap made the appointment because it is instigating a new way of working with agencies that requires its creative agencies to tackle different creative briefs for each of its Big City stations.
The Big City Network consists of Key 103 in Manchester, Metro Radio in Newcastle, Radio Aire in Leeds, Radio City in Liverpool, Hallam FM in Sheffield, Viking FM in Hull and Rock FM in Preston.
Karmarama's appointment does not affect Big City's communications planning account, held by Naked Communications, or its brand planning account, held by Sparkler.
Brendan Moffett, the brand director for Big City Network, said the change was made because the needs of the business had moved on: "We will be creating eight separate creative briefs and Karmarama liked the idea that it would be presented with this challenging set of briefs. We needed a real sparky creative shop to deliver engaging advertising."
Faulds had held the creative account for three years. The new way of working will involve Sparkler developing local insights into each of the eight markets; Naked providing the media planning and then Karmarama developing creative.
Naresh Ramchandani, the joint creative director of Karmarama, said: "It's a great opportunity to deliver some unexpected creative solutions through a series of high- profile promotions."