Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the summer of 1987. This unprecedented event spanned 3 yr, affected 40 km² of seagrass and resulted in more than a decade of ecological disturbances. Initial putative causes for seagrass die-off ranged from climatic anomalies and watershed changes to wasting disease and eutrophication. Subsequent experimental research suggested that hypoxic plant tissue, caused by low water column oxygen content or reduced photosynthesis, allowed intrusion of sulfide leading to plant death. Contributing factors included high temperatures, salinities and T. testudinum biomass, together causing lower oxygen water solubility, higher community respiration rates and elevated nighttime oxygen demand. The Fisheries Habitat Assessment Program (FHAP) has tracked the system’s slow recovery since 1995. Recent FHAP data (2012) indicated that T. testudinum had returned to pre-die-off densities in even the most severely affected locations. During the summer of 2015, following several months of drought, National Park Service researchers reported hypersaline conditions and a recurrence of seagrass die-off in north-central Florida Bay. An interagency effort is presently underway to document the duration, extent, impacts and possible factors responsible for the current mortality. Initial field surveys indicate that there is high spatial coincidence between the current and the 1987–1990 events and that hypersalinity, water column stratification and bottom-water anoxia might have once again resulted in mass mortality of T. testudinum in Florida Bay. The goal of this report is to alert the scientific community to the recurrence of this important ecological event.