Lovecraftian horror, also called cosmic horror or eldritch horror, is a subgenre of horror, fantasy fiction, and weird fiction that emphasizes the horror of the unknowable and incomprehensible more than gore or other elements of shock. It is named after American author H. P. Lovecraft (1890–1937). His work emphasizes things that are strange and eldritch, with themes of cosmic dread, forbidden and dangerous knowledge, madness, non-human influences on humanity, religion and superstition, fate and inevitability, and the risks associated with scientific discoveries, which are now associated with Lovecraftian horror as a subgenre. The cosmic themes of Lovecraftian horror can also be found in other media, notably horror films, horror games, and comics.

H. P. Lovecraft refined this style of storytelling into his own mythos that involved a set of weird, pre-human, and extraterrestrial elements. His work was influenced by authors such as Edgar Allan Poe, Algernon Blackwood, Ambrose Bierce, Arthur Machen, Robert W. Chambers, and Lord Dunsany. However, Lovecraft was keen to distinguish his work from existing gothic and supernatural fiction, elevating the horror, in his own words, to a "cosmic" level. Stephen King has said the best of Lovecraft's works are "uniquely terrible in all of American literature, and survive with all their power intact."

The hallmark of Lovecraft's work is cosmicism, the sense that ordinary life is a thin shell over a reality that is so alien and abstract in comparison that merely contemplating it would damage the sanity of the ordinary person, insignificance and powerlessness at the cosmic scale, and uncompromising negativity. Author China Miéville notes that "Lovecraft's horror is not one of intrusion but of realization. The world has always been implacably bleak; the horror lies in our acknowledging that fact." Many of Lovecraft's stories are set in New England.

The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.

Attack the story like a radiant suicide, utter the great NO to life without weakness; then you will see a magnificent cathedral, and your senses, vectors of unutterable derangement, will map out an integral delirium that will be lost in the unnameable architecture of time.

The core themes and atmosphere of cosmic horror were laid out by Lovecraft himself in "Supernatural Horror in Literature", his essay on gothic, weird, and horror fiction. A number of characteristics have been identified as being associated with Lovecraftian horror:

Much of Lovecraft's influence is secondary, as he was a friend, inspiration, and correspondent to many authors who developed their own notable works. Many of these writers also worked with Lovecraft on jointly written stories. His more famous friends and collaborators include Robert Bloch, author of Psycho; Robert E. Howard, creator of Conan the Barbarian; and August Derleth, who focused on extending the Cthulhu Mythos.

Subsequent horror writers also heavily drew on Lovecraft's work. While many made direct references to elements of Lovecraft's mythos, either to draw on its associations or to acknowledge his influence, many others drew on the feel and tone of his work without specifically referring to mythos elements. Some have said that Lovecraft, along with Edgar Allan Poe, is the most influential author on modern horror. Author Stephen King has said: "Now that time has given us some perspective on his work, I think it is beyond doubt that H. P. Lovecraft has yet to be surpassed as the Twentieth Century's greatest practitioner of the classic horror tale."