Its southern tip in the district of Tunaberg is the southernmost point of mainland Svealand, one of the three original crowns forming Sweden.

Politically, Nyköping is historically dominated by the Social Democrats, although margins between the coalitions have been closer in some recent elections.

Snow cover in winter is variable and by contrast, especially seaside summer nights are warm by Nordic standards.

In the rural part of the municipality, the majority live in the many inland villages where Stigtomta is the largest.

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 Hummelvik, Horn, Skeppsvik, Svärdsklova, Uttervik, and Örstig.

Tessin was a high school noted for the strength of its curricula in humanities, arts, and social studies.

The merger between the two organizations eventually led to an expanded Gripen being the sole public high school in the municipality.

Nyköpingshus, a medieval castle, draws many thousands of tourists yearly, especially for its summer outdoor play, Nyköpings gästabud, dramatizing 14th-century events in which king Birger Magnusson and Queen Märta invited his two brothers Valdemar and Erik to the castle, ostensibly for a banquet of reconciliation after a civil war in which Valdemar and Erik had once imprisoned Birger.

During the banquet at Nyköpingshus, Birger arrested Valdemar and Erik and committed them to the dungeon, where, according to legend, they starved to death.