Frank Belknap Long Jr. was an American writer of horror fiction, fantasy, science fiction, poetry, gothic romance, comic books, and non-fiction. Though his writing career spanned seven decades, he is best known for his horror and science fiction short stories, including contributions to the Cthulhu Mythos alongside his friend, H. P. Lovecraft. During his life, Long received the World Fantasy Award for Life Achievement, the Bram Stoker Award for Lifetime Achievement, and the First Fandom Hall of Fame Award (1977).

The Cthulhu Mythos is a mythopoeia and a shared fictional universe, originating in the works of American horror writer H. P. Lovecraft. The term was coined by August Derleth, a contemporary correspondent and protégé of Lovecraft, to identify the settings, tropes, and lore that were employed by Lovecraft and his literary successors. The name "Cthulhu" derives from the central creature in Lovecraft's seminal short story "The Call of Cthulhu", first published in the pulp magazine Weird Tales in 1928.