TNS has held a majority interest in TNS InterScience since 2005 and now owns 100% of the company.
WPP said that the investment continues its strategy of developing its networks in faster-growing markets and sectors.
Founded in 1983, TNS InterScience is based in Sao Paulo and employs 100 people. Its clients include more than half of TNS' global account list, including Kraft, P&G, Telefonica and Samsung.
The agency, which specialises in the FMCG, technology, finance and automotive sectors, had revenues of 30m Brazilian Reais (£8.1m) for the year ending December 31 2007.
Last month WPP signed an agreement with the Nielsen Company to swap its 50% share of AGB Nielsen Media Research for three of Nielsen's assets.
The deal allowed WPP to satisfy regulatory requirements that it must dispose of its stake in AGB Nielsen Media Research in order to complete its acquisition of TNS.