Södermanland (Swedish pronunciation: [ˈsø̂ːdɛrmanˌland] or [ˈsø̌ː-] ^ⓘ), locally Sörmland, sometimes referred to under its Latinized form Sudermannia or Sudermania, is a historical province (or landskap) 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.

Södermanland means "(The) Land of the Southern Men", where the "southern men" (södermännen) were the people living south of Uppland.

The traditional provinces of Sweden serve no administrative or political purposes, but are historical and cultural entities. There is a corresponding administrative Södermanland County. However, the bulk of the population is within Stockholm County.

The coat of arms was granted in 1560. The arms is represented with a ducal coronet. Blazon: "Or, a Griffin rampant Sable beaked, langued, membered and armed Gules." The same CoA was granted for the county in 1940.

Södermanland is situated between lake Mälaren to the north and the Baltic Sea to the south and east. In the south, the border to Östergötland is comprised by the Kolmården forest.

The terrain is flat, with its highest altitude being Skogsbyås at 124 m (407 ft) elevation. The terrain largely consists of water filled hollows covered with woods on the heights.

There are three major water regions. One in the west, where lake Hjälmaren drains into Mälaren. There is a second water region on Södertörn, which is rather small in size and has no larger lakes. Smaller lakes in that region include Bornsjön (6.5 km² (2.5 sq mi)), Orlången (2.5 km² (0.97 sq mi)), and Magelungen (2.9 km² (1.1 sq mi)) as well as Naten lake. The third region is to the south towards Kolmården.

Both the city of Stockholm and Stockholm County are divided in the middle between Södermanland (southern part) and Uppland (northern part).