Landis, which was founded over 30 years ago, posted revenues of $9.3m (£4.7m) for the year ended December 31 2006; gross assets at that date were $1.9m.
The company's co-chairmen, Steve Landis and Robert Posten, will remain with Landis after the sale.
TNS said Landis would give it complete coverage of the entire cycle of services required by clients, from concept and product development to sales forecasting and launch tracking.
David Lowden, chief executive of TNS, said: "New product development is a key business area for TNS and we are one of the world's biggest players in this area.
"Landis is a top product innovation firm and this acquisition gives us full coverage in the area of new product development in our North America Custom business and across the group's global network.
"It will allow us to deliver additional insights to clients, especially in our Consumer and Healthcare sectors."