Salt marshes in the southeastern United States have recently experienced massive die-off, one of many examples of widespread degradation in marine and coastal ecosystems. Although intense drought is thought to be the primary cause of this die-off, we found snail grazing to be a major contributing factor. Survey of marsh die-off areas in three states revealed high-density fronts of snails on die-off edges at 11 of 12 sites. Exclusion experiments demonstrated that snails actively converted marshes to exposed mudflats. Salt addition and comparative field studies suggest that drought-induced stress and grazers acted synergistically and to varying degrees to cause initial plant death. After these disturbances, snail fronts formed on die-off edges and subsequently propagated through healthy marsh, leading to cascading vegetation loss. These results, combined with model analyses, reveal strong interactions between increasing climatic stress and grazer pressure, both potentially related to human environmental impacts, which amplify the likelihood and intensity of runaway collapse in these coastal systems.