Södermalm, often shortened to "Söder", is a district in central Stockholm. It covers the large island formerly called "Åsön". With a population of 99,685, it is one of the most densely populated districts of Scandinavia. Although Södermalm usually is considered an island, water to both its north and south does not flow freely but passes through locks.

Södermalm is connected to its surrounding areas by a number of bridges. It connects to Gamla Stan to the north by Slussen, a grid of road and rail and a lock that separates the lake Mälaren from the Baltic Sea, to Långholmen and Kungsholmen to the northwest by one of the city's larger bridges, Västerbron, to the islet Reimersholme to the west, to Liljeholmen to the southwest by the bridge Liljeholmsbron, to Årsta by Årstabron and Skanstullsbron in the south, and, finally, to Södra Hammarbyhamnen to the east by Danvikstull Bridge.

Administratively, Södermalm is part of Stockholm Municipality. It constitutes, together with Gamla stan and some other districts, from 2007 the administrative district Södermalms stadsdelsområde, often translated as Södermalm borough.