East Beleriand was the part of Beleriand that lay east of the River Sirion, and west of the Gelion. This region was ruled by Thingol from Doriath and (east of Aros) largely under the control of the Sons of Fëanor during the First Age.
Sirion descended from the north to the south. With its tributaries Mindeb, Esgalduin, Aros and its tributary Celon, they formed Neldoreth and Region, the forests of Doriath. North of Doriath, the rivers separated the lands of Dimbar, Nan Dungortheb, Dor Dinen and Himlad, just south from the northern highlands.
The eastern part of Beleriand was dominated by the river Gelion, which separated the main lands from Thargelion and the land of Ossiriand, marked by the river's seven tributaries (Ascar, Thalos, Legolin, Brilthor, Duilwen and Adurant). Further to the east, the Blue Mountains separated Beleriand from Eriador.
Between the great rivers Sirion and Gelion, to the south of Beleriand was the great forest Taur-im-Duinath.