In J. R. R. Tolkien's fictional Middle-earth, the Anduin or Great River of Wilderland is the longest river in the Third Age (the original Sindarin name means Long River), rising east of the Misty Mountains and flowing south through Wilderland and eastern Gondor. Gondor's ancient capital Osgiliath straddles the river; after the fall of Osgiliath the river effectively marks the eastern limit of Gondor's influence. South of Osgiliath the river turns south west, past the harbour of Harlond close to the Rammas Echor south of Minas Tirith (Barbara Strachey, in Journeys of Frodo, places the harbour just outside the wall), and flows out of a broad delta to the Bay of Belfalas and the Sundering Sea.

It was in the Gladden Fields in the northern reaches of Anduin that Isildur was slain and the One Ring lost; and it was there, more than two millennia later, that Déagol found the Ring and Sméagol took it from him.

The Anduin begins as two different streams in the Ered Mithrin, both called "Langwell" by the Éothéod when they lived in the triangle of land formed by it. Their old capital Framsburg was built at the confluence of these streams where the Anduin proper began.