Underground rivers are rivers that are partially or entirely subterranean. They may contain rapids, underground lakes, and underground waterfalls. They may run through flooded chambers and empty into the ocean via sea caves. Underground rivers are frequent elements of fantasy worlds and underworld locations. They may act as ferry routes and may be secret passageways through mountain ranges or other areas. In desert areas underground rivers may play important roles in local ecology.

When designing underground rivers for fictional worlds, consider their route through the subterranean space. These rivers may pass through a number of different chambers, offering worldbuilders a chance to explore the dark underground.