Nick Hiddleston is due to start at ZenithOptimedia next month.
He takes over from Anthony Jones who left the Publicis agency in June to move to Carat International as global head of its Insight division.
In his new role as worldwide research director, Hiddleston will report to Frank Harrison, ZenithOptimedia's worldwide strategic resource director and his responsibilities will include developing worldwide research systems and supporting the agency's international division.
Harrison said that Hiddleston's experience in marketing measurement would be a key factor in ZenithOptimedia delivering return on investment.
"Hiddleston is one of the top researchers in our business anywhere in the world and brings a lot of experience that will prove very useful in what we are trying to achieve," he said.
"It's a large, challenging role, but the reality of agencies today is that research and systems have a major part to play in the product proposition and Nick will be beefing up our capability in that area, which is very good news."
Hiddleston said that he was "delighted to be going to such a great, growing company".
Hiddleston's departure from Initiative comes at a period of flux for the agency, which earlier this year lost its boss in Europe, Asia Pacific and Latin America, Marie-José Forissier. She had agreed to step down.
Parent company Interpublic has also been through its own upheaval of senior management which ended last week with the appointment of Nick Brien as the new worldwide chief executive and president of its Universal McCann media planning and buying shop.
An Initiative spokesman confirmed Hiddleston's departure and wished him well, adding that the recruitment process for a replacement was already underway.
She continued: "We plan to expand the international research team with the addition of a new manger and a new graduate – it is very much going forward rather than downsizing."
As part of the changes, Tonya Perretta, head of digital futures, has been promoted to head Initiative's emerging and interactive media offering, Code – which stands for "creating order in the digital era".