The lost world is a subgenre of the fantasy or science fiction genre that involves the discovery of a new world out of time, place, or both. It began as a subgenre of the late-Victorian imperial romance and remains popular into the 21st century.

The genre arose during an era when the fascinating remnants of lost civilizations around the world were being discovered, such as the tombs of Egypt's Valley of the Kings, the semi-mythical stronghold of Troy, the jungle-shrouded pyramids of the Maya, or the cities and palaces of the empire of Assyria. Thus, real stories of archaeological finds by imperial adventurers succeeded in capturing the public's imagination. Between 1871 and the First World War, the number of published lost-world narratives, set in every continent, dramatically increased.

The genre has similar themes to "mythical kingdoms", such as El Dorado.