The Great Galactic Ghoul is a fictional space monster that consumes Mars probes. It was coined as a humorous attempt to explain the high failure rate of probes sent to that planet; of approximately thirty probes, two thirds have failed - sometimes for unclear reasons. The Galactic Ghoul seems to particularly enjoy the taste of Soviet and Russian probes; indeed, the concept of the Galactic Ghoul appears to have originated with frustrated Soviet space scientists.

In reality, the failure of so many probes is not particularly unexpected; space is a very harsh environment, and probes must function unmaintained for years to reach their destinations.

Some people believe that the ghoul is actually a group of intermediary aliens who are trying to protect Mars from Earth probes that have unfriendly motives.