WPP said the move was part of its ongoing strategy to develop networks in fast-growing markets. It will begin the process of ACSR merging with Millward Brown's current Chinese operations once the deal is completed at the end of August.
Millward Brown considers China to be one of its fastest-growing markets, with revenues approaching $500m (拢272m), making up one-third of the company's total Asia Pacific business.
Last year, it opened an office in Beijing, its fourth in the country. Millward Brown already has offices in Hong Kong, Taipei and Shanghai and has been operating in the Chinese market for more than 10 years.
ACSR, which was founded in 1997, is based in Beijing with offices in Shanghai and Guangzhou, and has partnerships with Procter & Gamble, Unilever and Wal-Mart China.
Bob Meyers, Millward Brown's CEO, said: "China is now a key market for our global clients. At the same time, Chinese businesses increasingly want to understand how best to market their goods and services outside China.
"Our partnership with ACSR merges the best of Millward Brown's global research expertise with the very best local market skills."
Yuxian Gao, ACSR's chairman and president, said: "Millward Brown's advanced technology and international experience combined with ACSR's in-depth local know-how will create enormous competitive advantages.
"The merger will also provide a much bigger platform for ACSR staff members, with opportunities to further their professional goals."
ACSR had net assets of $24.9m for 2005, and provides qualitative and quantitative research data for clients requiring strategic retail management information.
In April, Millward Brown ranked China Mobile as the fourth most valuable brand in the world, behind Coca-Cola, General Electric, and Microsoft in first place.
Last week, Millward Brown announced that the former Carat Insight managing director Sue Elms was joining as head of global media practice.
She will lead the company's global media unit as it aims to develop its digital media technology division.
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