Mystara is a campaign setting for Dungeons & Dragons. It originated as the Known World, a semi-generic setting used in early adventure modules, which was expanded upon in various D&D modules and sources.

Unlike most worlds, Mystara is a hollow planet, with a habitable surface on its interior called The Hollow World (surprisingly enough). This world is lit by an eternal red sun at the centre of Mystara, and serves as a 'cultural museum', preserving the societies that have become extinct in the outer world. The existence of the Hollow World is not, in general, known to the inhabitants of the Known World.

Mystara also incorporated the Blackmoor setting by describing Blackmoor as a highly advanced civilization that existed in the distant past.

Mystara's outer surface consists of 3 principal land masses; the continents of Brun, Skothar, and Davinia.

The inhabitants of mystara are diverse: humans of all races can be found here, along with myriad creatures such as elves, dwarves, halflings, orcs, dragons,and so on and so forth.

Some of the notable nations of Mystara include the Empire of Thyatis, the Kingdom of Karameikos, the Principalities of Glantri and the mysterious Alphatian Empire.

Mystara includes the Savage Coast, an area under the Red Curse , which eventually kills its inhabitants by mutating them unless the metal cinnabryl is worn in contact with the body. This area was published in its own module entitled Red Steel .