The Hitchhiker's Guide to the Galaxy is the first book in the Hitchhiker's Guide to the Galaxy comedy science fiction "trilogy of five books" by Douglas Adams, with a sixth book written by Eoin Colfer. The novel is an adaptation of the first four parts of Adams's radio series of the same name, centering on the adventures of the only man to survive the destruction of Earth; while roaming outer space, he comes to learn the truth behind Earth's existence. The novel was first published in London on 12 October 1979. It sold 250,000 copies in the first three months.

The Hitchhiker's Guide to the Galaxy is a comic science fiction series created by Douglas Adams that has become popular among fans of the genre and members of the scientific community. Phrases from it are widely recognised and often used in reference to, but outside the context of, the source material. Many writers on popular science, such as Fred Alan Wolf, Paul Davies, and Michio Kaku, have used quotations in their books to illustrate facts about cosmology or philosophy.