The Ruins of Undermountain is a boxed set for the Forgotten Realms campaign setting for the second edition of the Advanced Dungeons & Dragons fantasy role-playing game. The set was written by Ed Greenwood and published by TSR. It featured box cover art by Brom. and was published in 1991.

Forgotten Realms is a campaign setting for the Dungeons & Dragons (D&D) fantasy role-playing game. Commonly referred to by players and game designers as "The Realms", it was created by game designer Ed Greenwood around 1967 as a setting for his childhood stories. Several years later, it was published for the D&D game as a series of magazine articles, and the first Realms game products were released in 1987. Role-playing game products have been produced for the setting ever since, in addition to novels, role-playing video game adaptations, comic books, and the film Dungeons & Dragons: Honor Among Thieves.