Duke UK, which handles UK accounts such as Nissan GB, Levi Strauss UK, The Body Shop, Disney and Universal Music, will now become part of Digitas London.
The move has been made to strengthen the network agency's presence in London.
Digitas is a large global business with a number of international clients and a strong US base but no real domestic presence in the UK.
James Sanderson, the president of Digitas London, said: "Since starting at Digitas at the end of last year, one of my briefs for the agency has been to put it back on the map in London.
"The best way to do that is roll your sleeves up and hit the new business hard. This is made much easier when you have more skills at your disposal and existing clients and case studies."
Andy Jackson, the online marketing manager for Nissan GB, added: "The great creative brains we have on the business will benefit from this broader relationship and our 2010 digital marketing communications will benefit from performance-driven creativity - which is the promise this bigger agency and network says it will deliver."
Duke's staff has already begun moving into the network agency's Howland Street offices, with the merger expected to be completed before the end of the month.