Mouths of Sirion is a fictional location in J. R. R. Tolkien's Middle-earth universe. It was the refuge of the remnants of Eldar and Edain of Beleriand after the Nirnaeth Arnoediad and the Sack of Menegroth.
The Mouths of Sirion were home to Sindar of Doriath and the Falas, and Ñoldor from Nargothrond and Gondolin, as well as the few survivors of the Three Houses of the Edain. Under Tuor and Idril of Gondolin the refugees mingled, and later under Tuor's son Eärendil Half-Elven, who married the Half-Elven Elwing, Dior's daughter and survivor of the Sack of Doriath. While they dwelt there the land became known as Arvernien.
Elwing had the Silmaril of Beren and Lúthien, and they believed under this jewel they prospered. The refugees built many ships, sailing also the Isle of Balar, where Círdan had fled to with the survivors of Eglarest and Brithombar. Elwing gave birth to Elros and Elrond there.
Eärendil built the ship Vingilótë, and sought for Valinor, to ask for the pardon of the Valar. He travelled all over Belegaer, but could not find Valinor. While he was away, the surviving Sons of Fëanor attacked the Mouths of Sirion to reclaim the Silmaril. They killed many people but were almost all slain themselves, save for Maedhros and Maglor. Elwing cast herself into the Sea with the Silmaril, but was saved by Ulmo and sent to Eärendil. Maedhros later repented, and raised Elros and Elrond as his own children.
During the War of Wrath, which began after Eärendil and Elwing had reached Valinor, the Mouths of Sirion was the site where the muster of the remaining Eldar and Edain took place. After the War the Mouths and the Isle of Balar were where the survivors were gathered, and many of them went to Lindon until the Elves were summoned to Valinor, and the Edain to the new isle of Númenor.