Arthórien was a region in eastern Doriath.
Arthórien was located between the rivers Aros and Celon, west of Estolad and south of Himlad. Although what appears to be the same region is also called Radhrim, it is unknown if these two regions were the same (or if one was situated within the other).
After Denethor was killed upon Amon Ereb in the First Battle of Beleriand, some of the Green-elves fled to Doriath, apparently seeking the protection of the Girdle of Melian. These elves, who became known as the Guest-elves, settled in Arthórien.