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).