Södermanland, locally Sörmland, sometimes referred to under its Latinized form Sudermannia or Sudermania, is a historical province on the south eastern coast of Sweden. It borders Östergötland, Närke, Västmanland and Uppland. It is also bounded by lake Mälaren and the Baltic Sea.

Mälaren, historically referred to as Lake Malar in English, is the third-largest freshwater lake in Sweden. Its area is 1,140 km2 and its greatest depth is 64 m. Mälaren spans 120 kilometers from east to west. The lake drains, from south-west to north-east, into the Baltic Sea through its natural outlets Norrström and Söderström and through the artificial Södertälje Canal and Hammarbyleden waterway. The easternmost bay of Mälaren, in central Stockholm, is called Riddarfjärden. The lake is located in Svealand and bounded by the provinces of Uppland, Södermanland and Västmanland. The two largest islands in Mälaren are Selaön (91 km2) and Svartsjölandet (79 km2). Mälaren is low-lying and mostly relatively shallow.