TNS did not disclose what it paid for the company, which made revenues of $5m for the 12 months ending September 30 2006.
Sorensen Associates' Herb Sorensen, founder and business head, will continue to work with the business now it has moved to TNS.
TNS has also set up a new global division, TNS Retail and Shopper Insights, to help it meet demand for shopper research and consultancy services across multiple markets.
The practice will work in conjunction with TNS Custom Operations and TNS Worldpanel to create what it described as a "continuous measurement of consumer purchasing behaviour".
The acquisition of Sorensen Associates is intended to increase TNS's knowledge of a "shopper journey" from before they enter a store until point of purchase.
David Lowden, chief executive of TNS, said: "Retailers and manufacturers feel their needs are not being met by current shopper research services.
"TNS has responded by strengthening its offering, allowing us to service the needs of the many large companies that are forming shopper insight groups."
Lowden added that TNS had created an excellent tool for the US market that would also work well across its global network.