Historically, it was the name for all the coastal areas of the Baltic Sea, including the eastern parts of lake Mälaren, belonging to Svealand. The name is derived from Roden, which is the coastal equivalent to inland Hundreds. When the king would issue a call to leidang, the Viking Age equivalent of military conscript service, a Roden district was responsible for raising a number of ships for the leidang navy.
Etymologically, Roden, or Roslagen, is the source of the Finnish and Estonian names for Sweden: Ruotsi and Rootsi. The background of the Russian term Rus', as in Kievan Rus', Ruthenia, and ultimately Russia, derives from the same source as in the Finnish and Estonian words and is believed to derive from Varangians, the people from Roslagen.
The area also gives its name to the endangered domesticated Roslag sheep, which originated from the area centuries ago.