Nynäshamn Municipality (Nynäshamns kommun) is a municipality in Stockholm County in east central Sweden. Its seat is located in the city of Nynäshamn.In 1974 the former City of Nynäshamn (itself instituted in 1946) was merged with Ösmo, from which it had been detached as a market town (köping) in 1911 and Sorunda, all in the Södertörn peninsula and adjacent islands.