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

The present municipality was created as recently as 1992, when Nyköping Municipality, which Trosa had been a part of since 1974, was split in three parts.

The municipality borders to Nyköping Municipality, Gnesta Municipality, and in the north to Södertälje Municipality in .

Trosa is known for its season markets, Trosa Market in the summer and the Harvest and Christmas markets during fall and winter. There is also the annual race of Trosa Stadslopp, when over 1000 people run through on the streets of Trosa.

During the summer the municipality's population increases from 10 000 to 45 000. Mostly because the area got a lot of summer cabins, but there is also a significant number of tourists. A lot of boat tourists is also to be seen in the harbor or on the many islands in the municipals archipelago.

Within the municipality there are four churches; of those the church in Trosa rural parish is from the 13th century; Västerljung's Church is also from the medieval age; and the Trosa church is one of few 18th century churches in the area.

These are the results of the Riksdag elections for Trosa Municipality since the inaugual election under its current entity in 1991. For the 1991 to 1998 results the exact numbers of the Sweden Democrats were not published by the SCB due to the party's small size nationally.

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