Nyköpingsån is a 150 km long river primarily flowing through Södermanland County in Sweden. The river is named after Nyköping, where it flows into the Baltic Sea.
Nyköpingsån is one of two rivers to flow into the harbour of Nyköping, the other being Kilaån that winds through the southwestern part of the municipality eastwards from Kolmården.
The highest source of the river is the small lake of Svartsjön in Hallsberg Municipality, Örebro County at 187 metres above sea level. The river flows out of several sizeable lakes in Södermanland\'s interior.
