Nyköping Municipality (Nyköpings kommun) is a municipality in Södermanland County in southeast Sweden. Its seat is located in the city of Nyköping. Its southern tip is the southernmost point of Svealand, one of the three original crowns forming Sweden.
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.