"The Sentinel" is a science fiction short story by British author Arthur C. Clarke, written in 1948 and first published in 1951.

It appears in the short story collections Expedition to Earth (1953), The Nine Billion Names of God (1967), and The Sentinel (1982), as well as in The Lost Worlds of 2001 (1972).

The sentinel's destruction—which humans could only accomplish by mastering space travel and atomic energy—will have caused its signals to cease, thereby alerting its creators to intelligent life having discovered it.

The story's plot and ideas were a significant influence on the development of the 1968 film 2001: A Space Odyssey by Stanley Kubrick and on the novel.

And the locale was moved from the Mare Crisium to the most spectacular of all lunar craters, Tycho—easily visible to the naked eye from Earth at Full Moon.