Nyköping Municipality (Swedish: Nyköpings kommun) is a municipality in Södermanland County in southeast Sweden. Its seat is located in the city of Nyköping, which is containing a majority of the residents. Its southern tip in the district of Tunaberg is the southernmost point of mainland Svealand, 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 between the historical rural districts of Jönåker in the west and Rönö in the east. The northwestern part of the Jönåker hundred was replaced by the southeastern part of Oppunda. The village of Jönåker itself became part of the municipality as a result of the mergers. Politically, Nyköping is historically dominated by the Social Democrats, although margins between the coalitions have been closer in some recent elections. Employment has transformed from industry to more of a commuter and service town in recent decades.

Nyköping itself is on a wide inlet and has a seasonal tourist port that often freezes in winter. The rural and rugged coastline is a distinct feature of the municipality. The local climate has four distinct seasons. Snow cover in winter is variable and by contrast, especially seaside summer nights are warm by Nordic standards. Nyköpingsån is the largest river running through the municipality, with its basin forming a lake district around the village of Vrena in the north with the lakes of Yngaren and Långhalsen dominating the landscape. The largest lake of the drainage basin is Båven somewhat further east. Kilaån and Svärtaån are the other sizeable river systems draining into the Baltic Sea within the municipality.

In the rural part of the municipality, the majority live in the many inland villages where Stigtomta is the largest. There are also several villages of the Kila valley to the west of town, with a total of about 4,400 people in the three districts following the lowest course of the river. Elsewhere, the municipality is sparsely populated. Tystberga is the main village of the rural eastern parts and Nävekvarn the main settlement of the southern coast, with both being in relative isolation.

The following localities are large enough as in having more than 200 inhabitants to qualify under the Swedish definition of a locality. Many of these localities are former seats of rural municipalities.

Of those, only Buskhyttan, Nävekvarn and Sjösa are either onshore or near-coastal settlements, the rest being in the municipality's interior. Nävekvarn is the only other settlement than Nyköping to maintain a harbour and a seaside environment.

In addition there are numerous smaller settlements not large enough to meet the classification. The most significant of those are Aspa, Bränn-Ekeby, Koppartorp, Lappetorp, Lid, Lästringe, Ripsa, Råby and Svärta. Seaside mixed residence/holiday home areas also include Helgö, Horn, Hummelvik, Skeppsvik, Svärdsklova, Uttervik, Vålarö and Örstig.

The western area of the rural part of the municipality is more densely populated than the eastern side, with Stigtomta and the tri-village conurbation of Bergshammar, Svalsta and Enstaberga being located on that side. Skavsta Airport is also located in this section of the municipality. In 2018, there were 8,699 Swedish citizens of voting age registered in rural electoral wards set west of National Road 53 that goes roughly through the centre of the municipality compared to only 4,362 east of the road. This meant that 2/3 of the rural populace was in the western portion of the municipality.