Nynäshamn is a town (pop. 13,000) in Södermanland, Sweden and the seat of Nynäshamn Municipality, Stockholm County.

The town was founded in the early 20th century as a port about 60 kms south of Stockholm. The railway to Stockholm was opened in 1901.

Nynäshamn is well-known for being one of the places on the Swedish mainland where ferries to the island Gotland depart. Several cruise ships on tours in the Baltic Sea also stay in the harbour, because they are too large to go into Stockholm.

Nynäshamn is served by Stockholm commuter rail and several bus routes.