"At the Mountains of Madness" is a science fiction-horror novella by American author H. P. Lovecraft, written in February/March 1931.

The story details the events of a disastrous expedition to Antarctica in September 1930, and what is found there by a group of explorers led by the narrator, Dr. William Dyer of Miskatonic University. Throughout the story, Dyer details a series of previously untold events in the hope of deterring another group of explorers who wish to return to the continent. These events include the discovery of an ancient civilization older than the human race, and realization of Earth's past told through various sculptures and murals.

The story was inspired by Lovecraft's interest in Antarctic exploration; the continent was still not fully explored in the 1930s. Lovecraft explicitly draws from Edgar Allan Poe's novel The Narrative of Arthur Gordon Pym of Nantucket, and he may have used other stories for inspiration. Many story elements, such as the formless "shoggoth", recur in other Lovecraft works. The story has been adapted and used for graphic novels, video games, and musical works.