The Sörmland Museum is the county museum of Södermanland County in Sweden. The institution is responsible for preservation and conducting research in the area of the cultural history and archaeology of the county, including the medieval city of Nyköping.
The museum houses temporary art exhibits and a permanent exhibit about the Middle Ages, but lacks a permanent archaeological exhibit.
The museum is located in the old castle, Nyköpingshus on the river in central Nyköping.