The move will allow TNS to help meet the fast-growing demand for usability advice and consultancy.
Eye Square specialises in web, product, advertising and shop usability. It uses technology it has developed to provide in-depth analysis of mouse movements, mouse clicks and eye-tracking data.
Arno Hummerston, managing director of TNS Global Interactive, said: "Usability is a key discipline in market research and is seeing growing demand in the US and a number of European countries.
"We want to position ourselves so that we benefit from that demand. We believe Eye Square's solutions and expertise provide exciting opportunities for a wide range of TNS clients worldwide because it is a tool that is all about delivering business insights."