Oxelösund Municipality (Oxelösunds kommun) is a municipality in Södermanland County, in central Sweden. It borders by land to Nyköping Municipality. In 1950 a the Nikolai Rural municipality was dissolved. One part formed the then City of Oxelösund, one of the last newly created cities of Sweden. The rest of Nikolai was incorporated into the neighbouring City of Nyköping. In 1971 the city became a unitary municipalty without addition of territory, and it is with 33.7 km² one of Sweden's smallest.