Mithril is a metal in J. R. R. Tolkien's Middle-earth fantasy writings. It is described as silvery and stronger than steel but much lighter in weight. The malleability, lack of tarnishing and use of the metal in jewelry suggest some similarity to the real-world metal platinum, while its strength and lightness suggest titanium. Aluminum has also been suggested as a possible identity for mithril. There is, however, no authorial indication that Tolkien had a real-world metal in mind. The author first wrote of it in The Lord of the Rings, and it is retrospectively mentioned in the third, revised edition of The Hobbit in 1966. In the first 1937 edition, the mail shirt given to Bilbo is described as being made of "silvered steel".

In The Lord of the Rings, Tolkien wrote that mithril is found only in Khazad-dûm (Moria) in Middle-earth, where it is mined by the Dwarves. In Unfinished Tales he wrote that it is also found in Númenor.

The name mithril comes from two words in Sindarin — mith, meaning "grey" or "mist", and ril meaning "glitter". The metal's Quenya name is mistarillë. Mithril was also called "true-silver" or "Moria-silver"; the Dwarves had their own secret name for it.