Nynäshamn is a locality and the seat of Nynäshamn Municipality, Stockholm County, Sweden with 13,510 inhabitants in 2010.While interest in the area as a potentially useful port grew from the mid 19th Century, it was only with the opening of the railway to Stockholm in 1901 that Nynäshamn started to develop.During the early 20th Century, Nynäshamn also became well known as a spa town, though most such facilities were closed down before the end of World War I.