The Hitchhiker's Guide to the Galaxy is a science fiction comedy radio series primarily written by Douglas Adams. It was originally broadcast in the United Kingdom by BBC Radio 4 in 1978, and afterwards the BBC World Service, National Public Radio in the US and CBC Radio in Canada. The series was the first radio comedy programme to be produced in stereo, and was innovative in its use of music and sound effects, winning a number of awards.

The series follows the adventures of hapless Englishman Arthur Dent and his friend Ford Prefect, an alien who writes for The Hitchhiker's Guide to the Galaxy, a pan-galactic encyclopaedia and travel guide. After Earth is destroyed in the first episode, Arthur and Ford find themselves aboard a stolen spaceship piloted by Zaphod Beeblebrox (Ford's semi-cousin and Galactic President), depressed robot Marvin, and Trillian, the only other human survivor of Earth's destruction.

A pilot programme was commissioned in March 1977, and was recorded by the end of the following June. A second series was commissioned in 1979, transmitted in 1980. Episodes of the first series were re-recorded for release on LP records and audio cassettes and Adams adapted the first series into a best-selling novel in 1979. After the 1980 transmissions of the second radio series, a second novel was published and the first series was adapted for television. This was followed by three further novels, a computer game, and various other media.

Adams considered writing a third radio series to be based on his novel Life, the Universe and Everything in 1993, but the project did not begin until after his death in 2001. Dirk Maggs, with whom Adams had discussed the new series, directed and co-produced the radio adaptation as well as adaptations of the remaining Hitchhiker's Guide novels So Long, and Thanks for All the Fish and Mostly Harmless. These became the third, fourth and fifth radio series, transmitted in 2004 and 2005. A sixth series, adapting Eoin Colfer's sixth part in the "trilogy", And Another Thing... was broadcast in March 2018.