Alison Parker, the managing director of Billco, and Jane Cunningham, the managing director of Tribal, will jointly hold day-to-day responsibility for the new company.
The agency will be able to offer clients a full spectrum of digital services, from online marketing and strategic direction through to web design and programming.
Parker said that the move was due largely to digital marketing being hit hard by the economic downturn: "One does everything one can in the current economic climate to make a stronger business. We are already working together on several clients and creating one agency is a natural progression, which will meet client demands."
Tribal and Billco have recently worked together on successful pitches, including one for MegaStar.
Cunningham added: "We succeeded with MegaStar because we delivered consumer understanding and a strategy behind our design proposals. We want to offer new-business prospects all the options available to them, so we can help them meet their business objectives effectively. This will include anything from media planning and buying to creative, design and build."
The new Tribal DDB, taking all staff from both companies, totals 45 people.