Digit, an online agency 51 per cent owned by WPP, will become part of JWT's profit and loss following the merger.
Andy Chambers, the Digit founding partner and managing director, will join the JWT management team as chief digital officer. The Digit joint creative directors, Mike Bennett and Daljit Singh, will also relocate to JWT's offices as part of a core team based at the agency. They will report to the JWT chief executive, Alison Burns.
The two agencies have a long history of collaboration, working together on accounts including Shell, Wilkinson Sword and Nestle.
Digit will continue to run its own accounts from its Spitalfields offices, although a JWT source said greater integration between the two marketing companies was likely in the future.
Chambers added: "It is natural for agencies that work well together to want to build stronger partnerships and broaden their respective offerings to clients. As digital increasingly infuses all media delivery, it is an obvious alliance for both of us."
Burns said: "I am over the moon that Andy and his team are going to become part of JWT London."
She added: "Digit@JWT will ensure digital becomes an even more integral and natural part of our offering."
The merger comes as JWT prepares for the arrival of Russell Ramsey as its executive creative director. Ramsey joins the agency from Bartle Bogle Hegarty, where he was the chief creative officer.