Shelob is a fictional monster in the form of a giant spider from J. R. R. Tolkien's The Lord of the Rings. Her lair lies in Cirith Ungol ("the pass of the spider") leading into Mordor. The creature Gollum deliberately leads the Hobbit protagonist Frodo there in hopes of recovering the One Ring by letting Shelob attack Frodo. The plan is foiled when Samwise Gamgee temporarily blinds Shelob with the Phial of Galadriel, and then severely wounds her with Frodo's Elvish sword, Sting.

Some scholars have stated that Shelob is in the literary tradition of female monsters. Others have interpreted her as symbolising a sexual threat, with multiple sexual allusions. Scholars have noted her opposition to the Elves, and in particular her adversary, Galadriel, whose light helps the hobbits to defeat her darkness.

Shelob's physical appearance in Peter Jackson's film trilogy was based on the New Zealand tunnel-web spider.

Shelob is described in The Two Towers as an "evil thing in spider-form...[the] last child of Ungoliant to trouble the unhappy world", living high in the Ephel Dúath mountains on the borders of Mordor. Although she resided in Mordor and was unrepentantly evil, she was independent of Sauron and his influence. Her exact size is not stated, but she is significantly larger than her descendants, the Great Spiders of Mirkwood, and her hobbit opponents. She has a powerful bite to inject her venom and paralyse or kill her victims. Her hide is tough enough to resist sword-strokes, and the strings of her webs are likewise resilient to ordinary blades, though the magical Sting manages to cut them. Her main weak point is her eyes, which can be easily harmed or blinded.

She is introduced as both evil and ancient: "But still she was there, who was there before Sauron, and before the first stone of Barad-dûr; and she served none but herself, drinking the blood of Elves and Men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness". Her descendants include the Giant Spiders of Mirkwood defeated by Bilbo Baggins in The Hobbit.

Shelob's lair was Torech Ungol, below Cirith Ungol ("Pass of the Spider"), along the path that the Hobbits Frodo Baggins and Sam Gamgee took into Mordor, where Shelob had encountered Gollum during his previous trip to Mordor, and he apparently worshipped her. The Orcs of the Tower of Cirith Ungol called her "Shelob the Great" and "Her Ladyship", and referred to Gollum as "Her Sneak". Sauron was aware of her existence, but left her alone as a useful guard on the pass, and occasionally fed prisoners to her. In the story, Gollum deliberately led Frodo and Sam into her lair, planning to recover the One Ring once she had consumed the hobbits. She cornered them; but Frodo used the Phial of Galadriel's light to drive her off, and used Sting to cut the webs blocking the tunnel. Gollum waylaid the pair and tried to strangle Sam, while Shelob paralysed Frodo; but Sam fought off Gollum and then wielded Sting against Shelob. Seeking to crush Sam, she instead impaled herself upon Sting; and, being evil, was nearly blinded by the Phial of Galadriel, containing pure light from the Silmarils; whereupon she fled. Her eventual fate, Tolkien mentions in passing, "this tale does not tell." Thinking Frodo dead, Sam took the Ring from his friend and left his body behind in a bid to finish the quest himself, but discovered by listening to a pair of Orcs that Frodo was alive but senseless, under a minor influence of venom.

As Tolkien admitted in a letter to his son, Shelob "is of course only 'she + lob'", lob being an archaic English word for spider, influenced by Old English loppe or "spider". The word is not related to "cob" nor "cobweb". Old English attercoppe (meaning "spider") is derived from atter meaning "poison" and coppe meaning "head". Tolkien used "attercop" as well as "cob" and "lob" in The Hobbit, where Bilbo Baggins sings songs taunting the giant spiders in Mirkwood: "Attercop, Attercop, Old Tomnoddy" and "Lazy Lob and Crazy Cob".

The critic Joyce Tally Lionarons writes that Tolkien constructs the Elves and the spiders such as Shelob as polar opposites, the Elves good and bright, the spiders evil and dark. Milbank writes more specifically that the ancient Shelob's adversary is another ancient female character, the elf-queen Galadriel. Galadriel both chooses not to be "She-who-must-be-obeyed" by rejecting Frodo's offer of the One Ring, and gives Frodo her light (the Phial of Galadriel) which enables the hobbits to defeat Shelob.