Oxelösund Municipality (Oxelösunds kommun) is a municipality in Södermanland County in southeast Sweden. Its seat is located in the city of Oxelösund. In 1950 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 municipality without addition of territory, and it is with its 35.75 square kilometres (13.80 sq mi) one of Sweden's smallest municipalities.