Khuzdul (pronounced [kʰuzˈdul]) is a fictional language created by J. R. R. Tolkien, one of the languages of Middle-earth, specifically the secret and private language of the Dwarves. He based its structure and phonology on Semitic languages, primarily Hebrew, with triconsonantal roots of words. Very little is known of the grammar.

Tolkien began developing Khuzdul before the publication in 1937 of The Hobbit, with some names appearing in the early versions of The Silmarillion. Tolkien based Khuzdul on Semitic languages, primarily Hebrew, featuring triconsonantal roots and similarities to Hebrew's phonology and morphology. Tolkien noted some similarities between Dwarves and Jews: both were "at once natives and aliens in their habitations, speaking the languages of the country, but with an accent due to their own private tongue…". Tolkien commented of the Dwarves that "their words are Semitic obviously, constructed to be Semitic."

Although a very limited vocabulary is known, Tolkien mentioned he had developed the language to a certain extent. A small amount of material on Khuzdul phonology and root modifications has survived; it is yet to be published.

In the fictional setting of Middle-earth, little is known of Khuzdul (once written Khuzdûl), the Dwarves kept it secret, except for place names and a few phrases such as their battle-cry and Balin's tomb inscription in Moria, which read respectively:

According to the Lhammas, Khuzdul is a language isolate, the sole member of the Aulëan language family, not related to the Oromëan languages spoken by Elves (all of which are akin to Quenya). Aulëan was named from the Dwarvish tradition that it had been devised by Aulë the Smith, the Vala who created the Dwarves. Later, Tolkien dropped the origins of Elvish being taught by Oromë, but kept the origins of Khuzdul the same. It is said in The Silmarillion that Aulë created the dwarves, and taught them "the language he had devised for them", making Khuzdul, both in fiction and reality, a constructed language.

Dwarves were unwilling to teach outsiders Khuzdul, even to their non-dwarf friends. Dwarves would speak the languages of the region "but with an accent due to their own private tongue...". Dwarves were however willing to reveal the names of places in Khuzdul, such as the names of the landmarks of Moria: "I know them and their names, for under them lies Khazad-dûm, the Dwarrowdelf... Yonder stands Barazinbar, the Redhorn...and beyond him are Silvertine and Cloudyhead:...that we call Zirakzigil and Bundushathûr."

Besides their aglâb, spoken tongue, the Dwarves used a sign language, or iglishmêk, which was also just as secretive as Khuzdul. According to The War of the Jewels, it was learned simultaneously with the aglâb from childhood. The Dwarvish sign language was much more varied between communities than Khuzdul, which remained "astonishingly uniform and unchanged both in time and in locality". Tolkien described its structure and use: "The component sign-elements of any such code were often so slight and so swift that they could hardly be detected, still less interpreted by uninitiated onlookers. As the Eldar eventually discovered in their dealings with the Naugrim, they could speak with their voices but at the same time by ‘gesture’ convey to their own folk modifications of what was being said. Or they could stand silent considering some proposition, and yet confer among themselves meanwhile."

Tolkien only gave a few examples of the Iglishmêk sign language in his unpublished notes. The command to "Listen!" involved a slight raising of both forefingers simultaneously. The acknowledgment "I am listening" involved a slight raising of the right-hand forefinger, followed by a similar raising of the left-hand forefinger.