Hidden civilizations are civilizations or factions which the greater world either never knew existed or thinks died out long ago. They may be the last remnants of an ancient civilization. Often these civilizations are in remote or inaccessible locations, such as on mountaintops, deep within jungles, in deserts, or underground. There may be legends surrounding their lands, such as it being cursed, which may help to keep the outside world at bay. Often in fictional worlds hidden civilizations are encountered when outsiders search for new trade routes, seek hidden treasure, or attempt to take shortcuts through new lands.

Hidden civilizations often exist in remote or inaccessible locations, such as deep in the ocean, beneath the earth, or in isolated regions of the world. This isolation allows them to either develop or preserve unique cultures, knowledge, or technologies uninfluenced by outside forces. Fictional hidden civilizations are often the possessors of advanced technology, powerful artifacts, or secret knowledge.

Hidden civilizations may be aware of the greater world and may have hidden themselves on purpose, preferring an isolationist approach. They may act as protectors of a fantastic land, item, or ancient secret.

When designing hidden civilizations, consider what drove them into hiding and/or and what elements have kept them hidden.

First contact between outsiders and a hidden civilization may be fraught with dangers.