A gelatinous cube is a fictional monster originally created for the Dungeons & Dragons role-playing game. It was originally a ten-foot cube of mindless, gelatinous matter that slides from one place to another, absorbing everything in its path; in the most recent edition of Dungeons & Dragons they are fifteen-foot cubes. Because of its obviously fantastic, non-realistic nature, the gelatinous cube is one of the most well-known monsters created especially for role-playing games. Although it is based upon famous fictional monsters (especially the movie The Blob), it exists primarily as a role-playing game monster, and not a monster taken from outside sources (such as many mythological monsters including the minotaur and dryad) and adapted to a role-playing setting.

Despite its popularity (or perhaps because of it), the gelatinous cube is also widely known as one of the sillier role-playing monsters. It is something of a commentary on the ubiquity of treasure-laden dungeons in the Dungeons & Dragons universe, as the cube is a creature specifically adapted to a dungeon ecosystem. 10 feet to the side, it travels through standard 10-foot by 10-foot dungeon corridors, cleaning up debris and redistributing treasure by excreting indigestible metal items.