Nyköping is a locality and the seat of Nyköping Municipality, Södermanland County, Sweden with 29,891 inhabitants as of 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. It forms a wider conurbation with neighbouring minor town Oxelösund 10 kilometres (6.2 mi) south of its outskirts.
Nyköping literally translates as Newmarket into English. The prefix Ny is translated as New and köping is an old Swedish word for a market place and a commonly used suffix for cities in the south central region of the country. The city is located near the open Baltic Sea coast, and is regarded as a coastal location. Nyköping is also the home of Stockholm Skavsta Airport which is located less than 10 kilometres (6.2 mi) from the city centre. Nyköping is part of the wider area of the Mälaren Valley, being around 100 kilometres (62 mi) from inner Stockholm. It retains an oceanic/continental climate hybrid, causing warm summers and snowy winters.