The media group acquired the stock of Washington-based GMI, a provider of online panels to the market research industry, through its wholly-owned Kantar business. GMI had revenues of $64m (£40m) last year.
The deal marks WPP’s twelfth acquisition this year, maintaining its focus on digital agencies.
It has also strengthened its position in fast growing markets, with deals in countries including Vietnam, Brazil, the Philippines and India, as well as developed markets such as Germany, Canada, the US and Japan.
WPP last month reported a 37 per cent surge in pre-tax profits but said it expects to feel the impact of the economic slowdown next year. It upgraded its 2011 outlook in April after it outperformed peers in the first quarter.
A company spokesman said: “This investment continues WPP’s strategy of investing in fast-growing markets and sectors and further strengthening the capabilities of Kantar, which is one of the world’s largest insight, information and consultancy groups.”