Trosa is a locality and the seat of Trosa Municipality, Södermanland County, Sweden with 5,027 inhabitants in 2010 (town) and 11,417 in 2010 (municipality). The municipality of Trosa consists of the three towns Trosa, Västerljung and Vagnhärad. Even though its small size, Trosa states its place in Sweden. It is the second fastest growing town in Sweden (2,9 %/year 2008) and is ranked as having the 3rd best business climate in Sweden (2011).
Trosa was known as a town in the 14th century. In the 15th century it was an important site for merchancy, and received its city privileges in 1454 by King Charles VIII of Sweden (a.k.a. Karl Knutsson Bonde). Due to land elevation, the city center was moved in the early 17th century, to its present location by the estuary of the Trosa River. The city gained its valour at the new location from King Karl IX, 19 april 1610.
The city was burnt down and plundered by Russians in 1719 and was repeatedly attacked during the Great Northern War, but the local church and poor-house was spared at every occasion. Both these buildings are now on display in the center of Trosa and serves as a memory from those hard times.