Prospera was established in 1985 and is based in Stockholm.
The acquisition will allow TNS to build further syndicated services in the finance sector and to extend these across the Nordic region and other markets in Europe.
Agneta Elvesdtedt will continue as managing director of TNS Prospera.
David Lowden, chief executive of TNS, said: "TNS already has a strong position in Sweden's finance sector. This acquisition makes TNS the market leader in research for commercial banks and insurance companies.
"It is also in line with the group's strategy to increase its proportion of syndicated services."
Prospera's gross assets at December 31 2007 were £0.48m.
Last month TNS acquired Irish media intelligence company MediaMarket. It also reported, in a trading update, that it expected underlying revenue growth for the group to be more than 5% and to experience good levels of new-business activity going into 2008.