Trosa Municipality is a municipality in Södermanland County in southeast Sweden. The municipality consists of the localities of Vagnhärad, Västerljung, Stensund, Sund and the town of Trosa. The seat of the municipality is located in the town of Trosa.

Södermanland County is a county or län on the southeast coast of Sweden. In the local Sörmlandic dialects it is virtually universally shortened and pronounced as Sörmlands län, or simply Sörmland, which is the dominant pronunciation and spelling inside the county. For example, the name of the local regional council is Region Sörmland. It borders the counties of Östergötland, Örebro, Västmanland, Uppsala, Stockholm and to the Baltic Sea. It holds the popular camping route called "Sörmlandsleden" which is a system of trails covering a total of approximately 100 mil (1000 km) of walking paths, in Södermanland. Prince Alexander, the son of Prince Carl Philip, is Duke of Södermanland.