The Great God Pan is a short horror and fiction book by Arthur Machen, who is Welsh. Machen saw and felt the ruins of a pagan temple in Wales, which made him want to write The Great God Pan. In 1890, the magazine The Whirlwind gave out what would become the first part of the story. Later, Machen incorporated additional elements into The Great God Pan, leading to its publication as a book in 1894, which also included the story "The Inmost Light." At the start of the story, a woman named Mary is put through an experiment that aims to help her see the supernatural. After this, there is a story about a woman named Helen Vaughan who was involved in several strange deaths and events over a long period. In the end, the heroes talk to Helen and make her kill herself. Before she dies, she goes through a series of strange changes that show her to be a magical being.

When the story came out, the press widely viewed it as degenerate and awful due to its apparent sexual content, which damaged Machen's reputation as an author. Critics re-evaluated Machen's work in the 1920s, establishing The Great God Pan as a classic in the horror genre. Literary experts have acknowledged the influence of other 1800s works on The Great God Pan, yet they disagree on its classification as Gothic or science fiction. The short story, which has spawned two plays, served as inspiration for horror writers such as Bram Stoker, H. P. Lovecraft, and Stephen King.