one of the three original crowns forming Sweden. Nyköping is the administrative capital of Södermanland and hosts both the governors' residence and the regional council.

The municipality was created in 1971 with the amalgamation of the City of Nyköping and a great number of other municipalities. It was split up in three parts in 1992, when Gnesta Municipality and Trosa Municipality were created. The rural areas of the modern municipality is a merger