A storm drain, storm sewer, stormwater drain (Australia) or surface water (UK) system is designed to drain excess rain and ground water from an area. Ideally storm drains should be separate from sanitary sewers, though in some places the runoff from storm drains is subjected to sewage treatment when there is sufficient capacity to spare. In these systems a sudden large rainfall that exceeeds sewage treatment capcity will be allowed to overflow directly from the storm drains into watersheds via structures called combined sewer overflows.

Storm drains are present on most motorways, freeways and other busy roads, as well as towns in areas which experience heavy rainfall, flooding and coastal towns which experience regular storms.

An international subculture has grown up around the exploration of stormwater drains. Societies such as the Cave Clan regularly explore the drains underneath cities; this is commonly known as urban exploration.