Nyköping is a locality and the seat of Nyköping Municipality, Södermanland County, Sweden with 29,891 inhabitants in 2010. The city is also the capital of Södermanland County. Including Arnö, the locality on the southern shore of the bay just a couple of kilometres from the city centre, Nyköping would have 33,762 inhabitants according to the same SCB source. Commonly, Arnö is referred to as a part of the city proper.

Nyköping is a locality and the seat of Nyköping Municipality, Södermanland County, Sweden with 29,891 inhabitants in 2010. The city is also the capital of Södermanland County. Including Arnö, the locality on the southern shore of the bay just a couple of kilometres from the city centre, Nyköping would have 33,762 inhabitants according to the same SCB source. Commonly, Arnö is referred to as a part of the city proper.