The Johnstown Flood (locally, the Great Flood of 1889) occurred on Friday, May 31, 1889, after the catastrophic failure of the South Fork Dam, located on the south fork of the Little Conemaugh River, 14 miles (23 km) upstream of the town of Johnstown, Pennsylvania, United States. The dam ruptured after several days of extremely heavy rainfall, releasing 14.55 million cubic meters of water. With a volumetric flow rate that temporarily equaled the average flow rate of the Mississippi River, the flood killed 2,209 people and accounted for $17 million of damage (about $490 million in 2020 dollars).

The American Red Cross, led by Clara Barton and with fifty volunteers, undertook a major disaster relief effort. Support for victims came from all over the U.S. and eighteen foreign countries. After the flood, survivors suffered a series of legal defeats in their attempts to recover damages from the dam's owners. Public indignation at that failure prompted the development in American law changing from a fault-based regime to one of strict liability.