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 Hummelvik, Horn, Skeppsvik, Svärdsklova, Uttervik, 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 rendered 2/3 of the rural populace was in the western portion of the municipality. The majority of residents, however, are living in Nyköping's urban area.
Nyköping is the site of the Nyköpings Gymnasium, consisting of two schools: Gripen and Tessin as well as a smaller one located within the Stockholm-Skavsta Airport. Tessin being a high school noted for the strength of its curricula in humanities, arts, and social studies. Although Gripen has a higher admission credit needed as it has the more desirable program. The highest admission score to Tessin was for the year 2011-12 175 points and for the same year the science program on Gripen needed a score of 225, keep in mind these scores could have decreased during the admission period as people might have gotten in as reserves to one of their higher choices.
Tourist attraction
, 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. Shortly thereafter supporters of Valdemar and Erik forced Birger and Märta to flee to Denmark.
Only parties that have won representation during this timeframe are listed in results. Sweden Democrats have their results from 1988 to 1998 listed as "0.0" even though those were decimals higher, since the SCB did not publish those results in detail at a municipal level since the party was not in realistic contention to enter the Riksdag. From 1991 Trosa Municipality and Gnesta Municipality had separate elections rending significant boundary changes.
