Nyköping () is a locality and the seat of Nyköping Municipality, Södermanland County, Sweden with 32,759 inhabitants as of 2017. 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 above 36,000 inhabitants. Commonly, Arnö is referred to as a part of the city proper. It forms a wider conurbation with the neighbouring minor municipality and town of Oxelösund south of its outskirts.