The US-based agency will be incorporated into WPP's information and consultancy group Kantar. The 25-year-old Ziment employs 350 staff and has five operating units across 12 offices. Most of its senior management are expected to remain with the company.
The acquisition will widen Kantar's services to clients in the pharmaceutical, biotechnology and medical device sectors. Last year, Kantar's revenues were just under $1bn (£705m), with more than 75% of that generated outside the US.
Howard Ziment, CEO of The Ziment Group, said the idea is to form global partnerships with clients. "By combining Ziment's healthcare market knowledge with Kantar's global research infrastructure that spans across 80 offices in 56 countries, we will truly be able to fulfil this promise," he said.