Mass mortalities of intertidal purple sea urchins, Strongylocentrotus purpuratus occurred at Malibu Lagoon State Beach, California, in 2010 and 2011. Both events followed the first heavy rain of the season, and coincided with the illegal breaching of a coastal lagoon. Osmotic shock from low-salinity lagoon water, the likely cause of death, may have acted jointly with stress from subaerial exposure during especially low tides. Massive die-offs of purple sea urchins have occurred at other localities, usually after natural conditions created lethal levels of osmotic or thermal stress, or because of human efforts to harvest or to eradicate the species. Annually recurring lagoon ruptures at Malibu, combined with predation by western gulls, can have a profound impact on the local population of S. purpuratus and on intertidal ecology.