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 was first mentioned in the year 1493 as "Rodzlagen". Before that the area was known as 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, Roden districts were responsible for raising a number of ships for the leidang navy.
The name comes from the rodslag, which is an old coastal Uppland word for a rowing crew of warrior oarsmen. Etymologically, Roden, or Roslagen, is the source of the Finnish and Estonian names for Sweden: Ruotsi and Rootsi.
A person from Roslagen is called a Rospigg which means "inhabitant of Ros". Swedes from the Roslagen area, that is "the people of Ros", gave their name to the Rus' people and thus to the states of Russia and Belarus (see Rus' (name)).
The area also gives its name to the endangered domesticated Roslag sheep, which originated from the area centuries ago. It is served by the Roslagsbanan, a narrow-gauge railway network from Stockholm.