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 Södertörnpeninsula and adjacent islands.
There are over 1,800 islands and islets in the municipality. Nynäshamn boasts sandy beaches, areas suitable for long walks, for riding horses and for bicycling, all popular activities. The municipality may have one of the most varying natures in the county, as it allows both activities related to the Stockholm archipelago, such as boat tours, as well as boggy grounds, agricultural areas, urban areas and even forests with elk and deer.
Its history is traced several thousands of years back. About 50 historical remains are considered notable enough to be marked on maps and attract visitors. They include both grave fields from the Iron Age, runestones from the Viking Age as well as newer curiosities such as 18th century farms and cottages.