"At the Mountains of Madness" is a novella by American author H.P. Lovecraft, written in 1931 and rejected that year by Weird Tales editor Farnsworth Wright on the grounds of its length. It was originally serialized in the February, March, and April 1936 issues of Astounding Stories. It has been reproduced in numerous collections.

The story details the events of a disastrous expedition to the Antarctic continent in September 1930, and what was 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.

The story has inadvertently popularized the concept of ancient astronauts, as well as Antarctica's place in the "ancient astronaut mythology".