Shelob is the giant spider Frodo and Sam encounter in J.R.R. Tolkien’s The Lord of the Rings.
Shelob lives in a tunnel near Cirith Ungol, the pass leading toward Mordor. Gollum guides Frodo and Sam into her lair, hoping that she will kill them so he can recover the One Ring. Shelob attacks Frodo, stings him, and leaves him apparently dead, but Sam drives her away with the Phial of Galadriel and Sting.
Shelob is more than an ordinary giant spider. Tolkien describes her as a descendant of Ungoliant, an ancient monstrous being associated with darkness and greed. This connection places Shelob within the much older mythology later collected in The Silmarillion.
A common confusion is between Shelob and the spiders of Mirkwood. Those spiders appear in The Hobbit and are descended from the same broad line of monstrous arachnids, but Shelob is a distinct character and lives in the Ephel Dúath beside Mordor.