In the fiction of J. R. R. Tolkien, Belegaer, the Great Sea or the Sundering Seas, is the sea of Arda that is west of Middle-earth.
The full extent of Belegaer is never made clear, but it reaches far enough to the north to be ice-covered, and far to the south.
Before the end of the Second Age, the land of Aman, home of the Valar, formed the western edge of Belegaer. Before the ruin of Beleriand at the end of the First Age, the sea was narrow and ice-filled in the north, forming the strait of Helcaraxë, the Grinding Ice. It was thus possible to cross from Aman to Middle-earth on foot, though with difficulty, as did Fingolfin and his people of the Noldor when fleeing Valinor.
At the end of the Second Age, when the seas were bent and the world was made round, Belegaer washed "new lands", and only the chosen could find the "Straight Road" to Valinor.
Belegaer has several islands and island chains:
- Tol Eressëa close to the shore of Aman
- the Twilit Isles or Enchanted Isles seaward of Tol Eressëa
- Númenor (Second Age only)
- the Western Isles (after the First Age)
- Tol Morwen
- Tol Fuin
- Himling
- Balar, in the Bay of Balar
- Tolfalas, in the Bay of Belfalas
- Bay of Eldamar
- Bay of Balar (First Age only)
- the Great Gulf (First Age only)
- Firth of Drengist (First Age only)
- Ice Bay of Forochel (after First Age)
- Gulf of Lune (after First Age)
- Bay of Belfalas (after First Age)
