Nyköping (nüˈchöˌpĭng) [key], city (1990 pop. 26,380), capital of Södermanland co., SE Sweden, a port on the Baltic Sea. It is a commercial and industrial center, producing flour, textiles, iron and brass, and furniture. An atomic research center is nearby. Nyköping was founded in the 13th cent. on the site of a former trading town. It was destroyed by fire in 1665, was rebuilt, and was sacked by the Russians in 1719. Historic structures in the city include St. Nicholas Church (13th–18th cent.), the city hall (17th cent.), and ruins of Nyköpingshus castle (13th cent.).