Nan-tathren, the "Land of Willows", was a region in Methiriad. It was located where the River Narog met the great River Sirion, a little to the north of the Mouths of Sirion.
Famed for its willows and its flowered meadows, Nan-tathren was a desolate land. The region played little part in the history of Arda, and was perhaps inhabited by Ents. The Land of Willows is said to be the place where butterflies of all kinds came into the world, and they remained common there during the First Age.
After Morgoth's triumph in the Nirnaeth Arnoediad, his Orcs and wolves are said to have roamed even the distant southern lands of Nan-tathren and Ossiriand, causing none of the Free peoples in Beleriand to be secure.
The refugees from Gondolin, led by Tuor, rested in Nan-tathren for a while on their long journey south from their city's ruin. They were healed of their wounds and sorrows by Ulmo's power, which was strong in the land from his presence in the River Sirion. There Tuor sang to his son Eärendil a song about his encounter with Ulmo, awakening in both the sea-longing.
In the late Third Age, Treebeard remembered Nan-tathren and sang about "the willow-meads of Tasarinan" to Merry and Pippin. Galadriel later expressed the hope that she and Treebeard might meet there again.