The merger will make Yankelovich Holdings part of WPP's research, insight and consultancy division, Kantar.
It will create a 145-strong research and consultancy business split between New York, London, Delhi, Mumbai and Chapel Hill North Carolina, where Yankelovich Holdings is based.
The American agency, which reported unaudited revenues of $17.8m for the year ending December 31 2006, specialises in research services for clients in the financial, media, telecommunications and healthcare sectors. It counts GlaxoSmithKline, Kraft and Unilever among its clients.
WPP said the merger of the two companies, which are currently undergoing plans to expand into China, fitted its strategy to develop measurement services for clients in important markets and sectors.