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. 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.